diff --git a/addons/web_editor/static/src/js/editor/odoo-editor/src/utils/utils.js b/addons/web_editor/static/src/js/editor/odoo-editor/src/utils/utils.js
index a61f35975a5..6293c74f913 100644
--- a/addons/web_editor/static/src/js/editor/odoo-editor/src/utils/utils.js
+++ b/addons/web_editor/static/src/js/editor/odoo-editor/src/utils/utils.js
@@ -1119,8 +1119,12 @@ export const formatSelection = (editor, formatName, {applyStyle, formatProps} =
         // Remove the format on all inline ancestors until a block or an element
         // with a class that is not related to font size (in case the formatting
         // comes from the class).
-        while (parentNode && (!isBlock(parentNode) && (parentNode.classList.length === 0 ||
-                [...parentNode.classList].every(cls => FONT_SIZE_CLASSES.includes(cls))))) {
+        while (
+            parentNode && !isBlock(parentNode) &&
+            !isUnbreakable(parentNode) && !isUnbreakable(currentNode) &&
+            (parentNode.classList.length === 0 ||
+                [...parentNode.classList].every(cls => FONT_SIZE_CLASSES.includes(cls)))
+        ) {
             const isUselessZws = parentNode.tagName === 'SPAN' &&
                 parentNode.hasAttribute('data-oe-zws-empty-inline') &&
                 parentNode.getAttributeNames().length === 1;
diff --git a/addons/web_editor/static/src/js/editor/odoo-editor/test/spec/format.test.js b/addons/web_editor/static/src/js/editor/odoo-editor/test/spec/format.test.js
index 7a3e25086b0..6728f485b87 100644
--- a/addons/web_editor/static/src/js/editor/odoo-editor/test/spec/format.test.js
+++ b/addons/web_editor/static/src/js/editor/odoo-editor/test/spec/format.test.js
@@ -882,6 +882,20 @@ describe('Format', () => {
                 contentAfter: '<table><tbody><tr><td><h1><span style="font-size: 18px;">[]<br></span></h1></td><td><h1><span style="font-size: 18px;"><br></span></h1></td></tr><tr><td><h1><br></h1></td><td><h1><br></h1></td></tr></tbody></table>',
             });
         });
+        it('should apply font size in unbreakable span with class', async () => {
+            await testEditor(BasicEditor, {
+                contentBefore: `<h1><span class="oe_unbreakable">some [text]</span></h1>`,
+                stepFunction: setFontSize('18px'),
+                contentAfter: `<h1><span class="oe_unbreakable">some <span style="font-size: 18px;">[text]</span></span></h1>`,
+            });
+        });
+        it('should apply font size in unbreakable span without class', async () => {
+            await testEditor(BasicEditor, {
+                contentBefore: `<h1><span t="unbreakable">some [text]</span></h1>`,
+                stepFunction: setFontSize('18px'),
+                contentAfter: `<h1><span t="unbreakable">some <span style="font-size: 18px;">[text]</span></span></h1>`,
+            });
+        });
     });
 
     it('should add style to a span parent of an inline', async () => {