diff --git a/ghost/email-service/lib/EmailRenderer.js b/ghost/email-service/lib/EmailRenderer.js
index cd2c3ef29209..8cf928b973c4 100644
--- a/ghost/email-service/lib/EmailRenderer.js
+++ b/ghost/email-service/lib/EmailRenderer.js
@@ -12,8 +12,10 @@ const {EmailAddressParser} = require('@tryghost/email-addresses');
 const {registerHelpers} = require('./helpers/register-helpers');
 const crypto = require('crypto');
 
+const DEFAULT_LOCALE = 'en-gb';
+
 // Wrapper function so that i18next-parser can find these strings
-const t = (x) => { 
+const t = (x) => {
     return x;
 };
 
@@ -38,10 +40,17 @@ function escapeHtml(unsafe) {
         .replace(/'/g, ''');
 }
 
-function formatDateLong(date, timezone, locale = 'en-gb') {
-    if (locale === 'en') { 
-        locale = 'en-gb'; 
+function isValidLocale(locale) {
+    try {
+        // Attempt to create a DateTimeFormat with the locale
+        new Intl.DateTimeFormat(locale);
+        return true; // No error means it's a valid locale
+    } catch (e) {
+        return false; // RangeError means invalid locale
     }
+}
+
+function formatDateLong(date, timezone, locale = DEFAULT_LOCALE) {
     return DateTime.fromJSDate(date).setZone(timezone).setLocale(locale).toLocaleString({
         year: 'numeric',
         month: 'long',
@@ -185,7 +194,7 @@ class EmailRenderer {
         this.#models = models;
         this.#t = t;
     }
-    
+
     getSubject(post, isTestEmail = false) {
         const subject = post.related('posts_meta')?.get('email_subject') || post.get('title');
         return isTestEmail ? `[TEST] ${subject}` : subject;
@@ -216,6 +225,21 @@ class EmailRenderer {
         };
     }
 
+    // Locale is user-input, so we need to ensure it's valid
+    #getValidLocale() {
+        let locale = this.#settingsCache.get('locale') || DEFAULT_LOCALE;
+
+        // Remove any trailing whitespace
+        locale = locale.trim();
+
+        // If the locale is just "en", or is not valid, revert to default
+        if (locale === 'en' || !isValidLocale(locale)) {
+            locale = DEFAULT_LOCALE;
+        }
+
+        return locale;
+    }
+
     getFromAddress(post, newsletter) {
         // Clean from address to ensure DMARC alignment
         const addresses = this.#emailAddressService.getAddress({
@@ -561,8 +585,8 @@ class EmailRenderer {
      * @returns {string}
      */
     getMemberStatusText(member) {
-        const t = this.#t; 
-        const locale = this.#settingsCache.get('locale');
+        const t = this.#t;
+        const locale = this.#getValidLocale();
 
         if (member.status === 'free') {
             // Not really used, but as a backup
@@ -625,7 +649,7 @@ class EmailRenderer {
      */
     buildReplacementDefinitions({html, newsletterUuid}) {
         const t = this.#t; // es-lint-disable-line no-shadow
-        const locale = this.#settingsCache.get('locale');
+        const locale = this.#getValidLocale();
 
         const baseDefinitions = [
             {
diff --git a/ghost/email-service/test/email-renderer.test.js b/ghost/email-service/test/email-renderer.test.js
index 5dd0018ab7cf..1afb5457e482 100644
--- a/ghost/email-service/test/email-renderer.test.js
+++ b/ghost/email-service/test/email-renderer.test.js
@@ -130,7 +130,7 @@ describe('Email renderer', function () {
                 email: 'test@example.com',
                 createdAt: new Date(2023, 2, 13, 12, 0),
                 status: 'free'
-            };  
+            };
         });
 
         it('returns the unsubscribe header replacement by default', function () {
@@ -388,7 +388,7 @@ describe('Email renderer', function () {
                 email: 'test@example.com',
                 createdAt: new Date(2023, 2, 13, 12, 0),
                 status: 'free'
-            };  
+            };
         });
 
         it('handles dates when the locale is en-gb (default)', function () {
@@ -399,6 +399,7 @@ describe('Email renderer', function () {
             assert.equal(replacements[0].id, 'created_at');
             assert.equal(replacements[0].getValue(member), '13 March 2023');
         });
+
         it('handles dates when the locale is fr', function () {
             emailRenderer = new EmailRenderer({
                 urlUtils: {
@@ -427,6 +428,7 @@ describe('Email renderer', function () {
             assert.equal(replacements[0].id, 'created_at');
             assert.equal(replacements[0].getValue(member), '13 mars 2023');
         });
+
         it('handles dates when the locale is en (US)', function () {
             emailRenderer = new EmailRenderer({
                 urlUtils: {
@@ -455,6 +457,65 @@ describe('Email renderer', function () {
             assert.equal(replacements[0].id, 'created_at');
             assert.equal(replacements[0].getValue(member), '13 March 2023');
         });
+
+        it('handles dates when the locale has whitespace like "en "', function () {
+            emailRenderer = new EmailRenderer({
+                urlUtils: {
+                    urlFor: () => 'http://example.com/subdirectory/'
+                },
+                labs: {
+                    isSet: () => labsEnabled
+                },
+                settingsCache: {
+                    get: (key) => {
+                        if (key === 'timezone') {
+                            return 'UTC';
+                        }
+                        if (key === 'locale') {
+                            return 'en ';
+                        }
+                    }
+                },
+                settingsHelpers: {getMembersValidationKey,createUnsubscribeUrl},
+                t: t
+            });
+            const html = '%%{created_at}%%';
+            const replacements = emailRenderer.buildReplacementDefinitions({html, newsletterUuid: newsletter.get('uuid')});
+            assert.equal(replacements.length, 2);
+            assert.equal(replacements[0].token.toString(), '/%%\\{created_at\\}%%/g');
+            assert.equal(replacements[0].id, 'created_at');
+            assert.equal(replacements[0].getValue(member), '13 March 2023');
+        });
+
+        it('handles dates when the locale is invalid like "(en)"', function () {
+            emailRenderer = new EmailRenderer({
+                urlUtils: {
+                    urlFor: () => 'http://example.com/subdirectory/'
+                },
+                labs: {
+                    isSet: () => labsEnabled
+                },
+                settingsCache: {
+                    get: (key) => {
+                        if (key === 'timezone') {
+                            return 'UTC';
+                        }
+                        if (key === 'locale') {
+                            return '(en)';
+                        }
+                    }
+                },
+                settingsHelpers: {getMembersValidationKey,createUnsubscribeUrl},
+                t: t
+            });
+
+            const html = '%%{created_at}%%';
+            const replacements = emailRenderer.buildReplacementDefinitions({html, newsletterUuid: newsletter.get('uuid')});
+            assert.equal(replacements.length, 2);
+            assert.equal(replacements[0].token.toString(), '/%%\\{created_at\\}%%/g');
+            assert.equal(replacements[0].id, 'created_at');
+            assert.equal(replacements[0].getValue(member), '13 March 2023');
+        });
     });
     describe('isMemberTrialing', function () {
         let emailRenderer;