diff --git a/docs/Methods.md b/docs/Methods.md
index b866eee..ab7b4df 100644
--- a/docs/Methods.md
+++ b/docs/Methods.md
@@ -26,27 +26,41 @@ Method responsible for configuring some of the library behaviours such as the gl
 
 ```ts
 interface InitOptions {
-  // the global fallback locale
-  fallbackLocale: string
-  // the app initial locale
-  initialLocale?: string
-  // custom time/date/number formats
-  formats?: Formats
-  // loading delay interval
-  loadingDelay?: number
+  /** The global fallback locale **/
+  fallbackLocale: string;
+  /** The app initial locale **/
+  initialLocale?: string | null;
+  /** Custom time/date/number formats **/
+  formats?: Formats;
+  /** Loading delay interval **/
+  loadingDelay?: number;
+  /**
+   * @deprecated Use `handleMissingMessage` instead.
+   * */
+  warnOnMissingMessages?: boolean;
+  /**
+   * Optional method that is executed whenever a message is missing.
+   * It may return a string to use as the fallback.
+   */
+  handleMissingMessage?: MissingKeyHandler;
+  /**
+   * Whether to treat HTML/XML tags as string literal instead of parsing them as tag token.
+   * When this is false we only allow simple tags without any attributes
+   * */
+  ignoreTag: boolean;
 }
 ```
 
 **Example**:
 
 ```js
-import { init } from 'svelte-i18n'
+import { init } from 'svelte-i18n';
 
 init({
   // fallback to en if current locale is not in the dictionary
   fallbackLocale: 'en',
   initialLocale: 'pt-br',
-})
+});
 ```
 
 ##### Custom formats
@@ -55,9 +69,9 @@ It's possible to define custom format styles via the `formats` property if you w
 
 ```ts
 interface Formats {
-  number: Record<string, Intl.NumberFormatOptions>
-  date: Record<string, Intl.DateTimeFormatOptions>
-  time: Record<string, Intl.DateTimeFormatOptions>
+  number: Record<string, Intl.NumberFormatOptions>;
+  date: Record<string, Intl.DateTimeFormatOptions>;
+  time: Record<string, Intl.DateTimeFormatOptions>;
 }
 ```
 
@@ -66,7 +80,7 @@ Please refer to the [Intl.NumberFormat](https://developer.mozilla.org/en-US/docs
 **Example**:
 
 ```js
-import { init } from 'svelte-i18n'
+import { init } from 'svelte-i18n';
 
 init({
   // fallback to en if current locale is not in the dictionary
@@ -76,13 +90,11 @@ init({
       EUR: { style: 'currency', currency: 'EUR' },
     },
   },
-})
+});
 ```
 
 ```html
-<div>
-  {$_.number(123456.789, { format: 'EUR' })}
-</div>
+<div>{$_.number(123456.789, { format: 'EUR' })}</div>
 <!-- 123.456,79 € -->
 ```
 
@@ -96,13 +108,13 @@ Utility method to help getting a initial locale based on a pattern of the curren
 **Example**:
 
 ```js
-import { init, getLocaleFromHostname } from 'svelte-i18n'
+import { init, getLocaleFromHostname } from 'svelte-i18n';
 
 init({
   // fallback to en if current locale is not in the dictionary
   fallbackLocale: 'en',
   initialLocale: getLocaleFromHostname(/^(.*?)\./),
-})
+});
 ```
 
 #### `getLocaleFromPathname`
@@ -116,13 +128,13 @@ Utility method to help getting a initial locale based on a pattern of the curren
 **Example**:
 
 ```js
-import { init, getLocaleFromPathname } from 'svelte-i18n'
+import { init, getLocaleFromPathname } from 'svelte-i18n';
 
 init({
   // fallback to en if current locale is not in the dictionary
   fallbackLocale: 'en',
   initialLocale: getLocaleFromPathname(/^\/(.*?)\//),
-})
+});
 ```
 
 #### `getLocaleFromNavigator`
@@ -136,13 +148,13 @@ Utility method to help getting a initial locale based on the browser's `navigato
 **Example**:
 
 ```js
-import { init, getLocaleFromNavigator } from 'svelte-i18n'
+import { init, getLocaleFromNavigator } from 'svelte-i18n';
 
 init({
   // fallback to en if current locale is not in the dictionary
   fallbackLocale: 'en',
   initialLocale: getLocaleFromNavigator(),
-})
+});
 ```
 
 #### `getLocaleFromQueryString`
@@ -154,13 +166,13 @@ init({
 Utility method to help getting a initial locale based on a query string value.
 
 ```js
-import { init, getLocaleFromQueryString } from 'svelte-i18n'
+import { init, getLocaleFromQueryString } from 'svelte-i18n';
 
 init({
   // fallback to en if current locale is not in the dictionary
   fallbackLocale: 'en',
   initialLocale: getLocaleFromQueryString('lang'),
-})
+});
 ```
 
 #### `getLocaleFromHash`
@@ -174,13 +186,13 @@ Utility method to help getting a initial locale based on a hash `{key}={value}`
 **Example**:
 
 ```js
-import { init, getLocaleFromHash } from 'svelte-i18n'
+import { init, getLocaleFromHash } from 'svelte-i18n';
 
 init({
   // fallback to en if current locale is not in the dictionary
   fallbackLocale: 'en',
   initialLocale: getLocaleFromHash('lang'),
-})
+});
 ```
 
 #### `addMessages`
@@ -224,10 +236,10 @@ Registers an async message `loader` for the specified `locale`. The loader queue
 **Example**:
 
 ```js
-import { register } from 'svelte-i18n'
+import { register } from 'svelte-i18n';
 
-register('en', () => import('./_locales/en.json'))
-register('pt', () => import('./_locales/pt.json'))
+register('en', () => import('./_locales/en.json'));
+register('pt', () => import('./_locales/pt.json'));
 ```
 
 See [how to asynchronously load dictionaries](/svelte-i18n/blob/master/docs#22-asynchronous).
diff --git a/src/runtime/configs.ts b/src/runtime/configs.ts
index 5501714..d7cef18 100644
--- a/src/runtime/configs.ts
+++ b/src/runtime/configs.ts
@@ -1,5 +1,10 @@
-import type { ConfigureOptions, ConfigureOptionsInit } from './types';
-import { $locale } from './stores/locale';
+import type {
+  ConfigureOptions,
+  ConfigureOptionsInit,
+  MissingKeyHandlerInput,
+} from './types';
+import { $locale, getCurrentLocale, getPossibleLocales } from './stores/locale';
+import { hasLocaleQueue } from './includes/loaderQueue';
 
 interface Formats {
   number: Record<string, any>;
@@ -38,11 +43,28 @@ export const defaultFormats: Formats = {
   },
 };
 
+/**
+ * Default missing key handler used in case "warnOnMissingMessages" is set to true.
+ */
+function defaultMissingKeyHandler({ locale, id }: MissingKeyHandlerInput) {
+  // istanbul ignore next
+  console.warn(
+    `[svelte-i18n] The message "${id}" was not found in "${getPossibleLocales(
+      locale,
+    ).join('", "')}".${
+      hasLocaleQueue(getCurrentLocale())
+        ? `\n\nNote: there are at least one loader still registered to this locale that wasn't executed.`
+        : ''
+    }`,
+  );
+}
+
 export const defaultOptions: ConfigureOptions = {
   fallbackLocale: null as any,
   loadingDelay: 200,
   formats: defaultFormats,
   warnOnMissingMessages: true,
+  handleMissingMessage: undefined,
   ignoreTag: true,
 };
 
@@ -57,6 +79,18 @@ export function init(opts: ConfigureOptionsInit) {
   // eslint-disable-next-line @typescript-eslint/prefer-nullish-coalescing
   const initialLocale = opts.initialLocale || opts.fallbackLocale;
 
+  if (rest.warnOnMissingMessages) {
+    delete rest.warnOnMissingMessages;
+
+    if (rest.handleMissingMessage == null) {
+      rest.handleMissingMessage = defaultMissingKeyHandler;
+    } else {
+      console.warn(
+        '[svelte-i18n] The "warnOnMissingMessages" option is deprecated. Please use the "handleMissingMessage" option instead.',
+      );
+    }
+  }
+
   Object.assign(options, rest, { initialLocale });
 
   if (formats) {
diff --git a/src/runtime/stores/formatters.ts b/src/runtime/stores/formatters.ts
index 18326c5..989f35b 100644
--- a/src/runtime/stores/formatters.ts
+++ b/src/runtime/stores/formatters.ts
@@ -9,7 +9,6 @@ import type {
   JSONGetter,
 } from '../types';
 import { lookup } from '../includes/lookup';
-import { hasLocaleQueue } from '../includes/loaderQueue';
 import {
   getMessageFormatter,
   getTimeFormatter,
@@ -18,7 +17,7 @@ import {
 } from '../includes/formatters';
 import { getOptions } from '../configs';
 import { $dictionary } from './dictionary';
-import { getCurrentLocale, getPossibleLocales, $locale } from './locale';
+import { getCurrentLocale, $locale } from './locale';
 
 const formatMessage: MessageFormatter = (id, options = {}) => {
   let messageObj = options as MessageObject;
@@ -43,20 +42,10 @@ const formatMessage: MessageFormatter = (id, options = {}) => {
   let message = lookup(id, locale);
 
   if (!message) {
-    if (getOptions().warnOnMissingMessages) {
-      // istanbul ignore next
-      console.warn(
-        `[svelte-i18n] The message "${id}" was not found in "${getPossibleLocales(
-          locale,
-        ).join('", "')}".${
-          hasLocaleQueue(getCurrentLocale())
-            ? `\n\nNote: there are at least one loader still registered to this locale that wasn't executed.`
-            : ''
-        }`,
-      );
-    }
-
-    message = defaultValue ?? id;
+    message =
+      getOptions().handleMissingMessage?.({ locale, id, defaultValue }) ??
+      defaultValue ??
+      id;
   } else if (typeof message !== 'string') {
     console.warn(
       `[svelte-i18n] Message with id "${id}" must be of type "string", found: "${typeof message}". Gettin its value through the "$format" method is deprecated; use the "json" method instead.`,
diff --git a/src/runtime/types/index.ts b/src/runtime/types/index.ts
index 16ee4b3..d579a27 100644
--- a/src/runtime/types/index.ts
+++ b/src/runtime/types/index.ts
@@ -72,12 +72,40 @@ export interface MessagesLoader {
   (): Promise<any>;
 }
 
+export type MissingKeyHandlerInput = {
+  locale: string;
+  id: string;
+  defaultValue: string | undefined;
+};
+
+export type MissingKeyHandlerOutput = string | void | undefined;
+
+export type MissingKeyHandler = (
+  input: MissingKeyHandlerInput,
+) => MissingKeyHandlerOutput;
+
 export interface ConfigureOptions {
+  /** The global fallback locale * */
   fallbackLocale: string;
+  /** The app initial locale * */
   initialLocale?: string | null;
+  /** Custom time/date/number formats * */
   formats: Formats;
+  /** Loading delay interval * */
   loadingDelay: number;
-  warnOnMissingMessages: boolean;
+  /**
+   * @deprecated Use `handleMissingMessage` instead.
+   * */
+  warnOnMissingMessages?: boolean;
+  /**
+   * Optional method that is executed whenever a message is missing.
+   * It may return a string to use as the fallback.
+   */
+  handleMissingMessage?: MissingKeyHandler;
+  /**
+   * Whether to treat HTML/XML tags as string literal instead of parsing them as tag token.
+   * When this is false we only allow simple tags without any attributes
+   * */
   ignoreTag: boolean;
 }
 
diff --git a/test/runtime/configs.test.ts b/test/runtime/configs.test.ts
index e182ad4..07a6484 100644
--- a/test/runtime/configs.test.ts
+++ b/test/runtime/configs.test.ts
@@ -1,20 +1,18 @@
 /* eslint-disable node/global-require */
 import { get } from 'svelte/store';
 
-import {
-  init,
-  getOptions,
-  defaultOptions,
-  defaultFormats,
-} from '../../src/runtime/configs';
+import { init, getOptions, defaultFormats } from '../../src/runtime/configs';
 import { $locale } from '../../src/runtime/stores/locale';
 
+const warnSpy = jest.spyOn(global.console, 'warn').mockImplementation();
+
 beforeEach(() => {
-  init(defaultOptions as any);
+  warnSpy.mockReset();
 });
 
 test('inits the fallback locale', () => {
   expect(getOptions().fallbackLocale).toBeNull();
+
   init({
     fallbackLocale: 'en',
   });
@@ -45,3 +43,20 @@ test('sets the minimum delay to set the loading store value', () => {
   init({ fallbackLocale: 'en', loadingDelay: 300 });
   expect(getOptions().loadingDelay).toBe(300);
 });
+
+test('defines default missing key handler if "warnOnMissingMessages" is "true"', () => {
+  init({ fallbackLocale: 'en', warnOnMissingMessages: true });
+  expect(typeof getOptions().handleMissingMessage).toBe('function');
+});
+
+test('warns about using deprecated "warnOnMissingMessages" alongside "handleMissingMessage"', () => {
+  init({
+    fallbackLocale: 'en',
+    warnOnMissingMessages: true,
+    handleMissingMessage() {},
+  });
+
+  expect(warnSpy).toHaveBeenCalledWith(
+    '[svelte-i18n] The "warnOnMissingMessages" option is deprecated. Please use the "handleMissingMessage" option instead.',
+  );
+});
diff --git a/test/runtime/stores/formatters.test.ts b/test/runtime/stores/formatters.test.ts
index f812e39..7882c5d 100644
--- a/test/runtime/stores/formatters.test.ts
+++ b/test/runtime/stores/formatters.test.ts
@@ -7,6 +7,7 @@ import type {
   TimeFormatter,
   DateFormatter,
   NumberFormatter,
+  MissingKeyHandler,
 } from '../../../src/runtime/types/index';
 import {
   $format,
@@ -39,12 +40,10 @@ addMessages('pt', require('../../fixtures/pt.json'));
 addMessages('pt-BR', require('../../fixtures/pt-BR.json'));
 addMessages('pt-PT', require('../../fixtures/pt-PT.json'));
 
-beforeEach(() => {
-  init({ fallbackLocale: 'en' });
-});
-
 describe('format message', () => {
   it('formats a message by its id and the current locale', () => {
+    init({ fallbackLocale: 'en' });
+
     expect(formatMessage({ id: 'form.field_1_name' })).toBe('Name');
   });
 
@@ -55,6 +54,8 @@ describe('format message', () => {
   });
 
   it('formats a message with interpolated values', () => {
+    init({ fallbackLocale: 'en' });
+
     expect(formatMessage({ id: 'photos', values: { n: 0 } })).toBe(
       'You have no photos.',
     );
@@ -67,6 +68,8 @@ describe('format message', () => {
   });
 
   it('formats the default value with interpolated values', () => {
+    init({ fallbackLocale: 'en' });
+
     expect(
       formatMessage({
         id: 'non-existent',
@@ -77,6 +80,8 @@ describe('format message', () => {
   });
 
   it('formats the key with interpolated values', () => {
+    init({ fallbackLocale: 'en' });
+
     expect(
       formatMessage({
         id: '{food}',
@@ -86,27 +91,38 @@ describe('format message', () => {
   });
 
   it('accepts a message id as first argument', () => {
+    init({ fallbackLocale: 'en' });
+
     expect(formatMessage('form.field_1_name')).toBe('Name');
   });
 
   it('accepts a message id as first argument and formatting options as second', () => {
+    init({ fallbackLocale: 'en' });
+
     expect(formatMessage('form.field_1_name', { locale: 'pt' })).toBe('Nome');
   });
 
   it('throws if no locale is set', () => {
+    init({ fallbackLocale: 'en' });
+
     $locale.set(null);
+
     expect(() => formatMessage('form.field_1_name')).toThrow(
       '[svelte-i18n] Cannot format a message without first setting the initial locale.',
     );
   });
 
   it('uses a missing message default value', () => {
+    init({ fallbackLocale: 'en' });
+
     expect(formatMessage('missing', { default: 'Missing Default' })).toBe(
       'Missing Default',
     );
   });
 
   it('errors out when value found is not string', () => {
+    init({ fallbackLocale: 'en' });
+
     const spy = jest.spyOn(global.console, 'warn').mockImplementation();
 
     expect(typeof formatMessage('form')).toBe('object');
@@ -117,7 +133,12 @@ describe('format message', () => {
     spy.mockRestore();
   });
 
-  it('warn on missing messages', () => {
+  it('warn on missing messages if "warnOnMissingMessages" is true', () => {
+    init({
+      fallbackLocale: 'en',
+      warnOnMissingMessages: true,
+    });
+
     const spy = jest.spyOn(global.console, 'warn').mockImplementation();
 
     formatMessage('missing');
@@ -129,7 +150,18 @@ describe('format message', () => {
     spy.mockRestore();
   });
 
+  it('uses result of handleMissingMessage handler', () => {
+    init({
+      fallbackLocale: 'en',
+      handleMissingMessage: () => 'from handler',
+    });
+
+    expect(formatMessage('should-default')).toBe('from handler');
+  });
+
   it('does not throw with invalid syntax', () => {
+    init({ fallbackLocale: 'en' });
+
     $locale.set('en');
     const spy = jest.spyOn(global.console, 'warn').mockImplementation();