diff --git a/phpBB/phpbb/template/twig/extension/icon.php b/phpBB/phpbb/template/twig/extension/icon.php
index f0748432a6..8edd33bbc7 100644
--- a/phpBB/phpbb/template/twig/extension/icon.php
+++ b/phpBB/phpbb/template/twig/extension/icon.php
@@ -83,7 +83,7 @@ class icon extends AbstractExtension
 		switch ($type)
 		{
 			case 'font':
-				// Nothing to do here..
+				$classes = $this->insert_fa_class($classes);
 			break;
 
 			case 'png':
@@ -168,6 +168,39 @@ class icon extends AbstractExtension
 		}
 	}
 
+	/**
+	 * Insert fa class into class string by checking if class string contains any fa classes
+	 *
+	 * @param string $class_string
+	 * @return string Updated class string or original class string if fa class is already set or string is empty
+	 */
+	protected function insert_fa_class(string $class_string): string
+	{
+		if (empty($class_string))
+		{
+			return $class_string;
+		}
+
+		// These also include pro class name we don't use, but handle them properly anyway
+		$fa_classes = ['fa-solid', 'fas', 'fa-regular', 'far', 'fal', 'fa-light', 'fab', 'fa-brands'];
+
+		// Split the class string into individual words
+		$icon_classes = explode(' ', $class_string);
+
+		// Check if the class string contains any of the fa classes, just return class string in that case
+		foreach ($icon_classes as $word)
+		{
+			if (in_array($word, $fa_classes))
+			{
+				return $class_string;
+			}
+		}
+
+		// If we reach this it means we didn't have any fa classes in the class string.
+		// Prepend class string with fas for fa-solid
+		return 'fas ' . $class_string;
+	}
+
 	/**
 	 * Prepare an SVG for usage in the template icon.
 	 *
diff --git a/tests/template/extension_test.php b/tests/template/extension_test.php
index 3050ed6c52..ea762f5b0f 100644
--- a/tests/template/extension_test.php
+++ b/tests/template/extension_test.php
@@ -313,7 +313,7 @@ class phpbb_template_extension_test extends phpbb_template_template_test_case
 				[
 					'ICON_PENCIL'	=> 'Pencil icon',
 				],
-				'<i class="o-icon o-icon-font fa-fw fa-pencil a-class another-class" title="Pencil icon" aria-hidden="true" data-attr-1="true" data-attr-2="two"></i>
+				'<i class="o-icon o-icon-font fa-fw fa-pencil fas a-class another-class" title="Pencil icon" aria-hidden="true" data-attr-1="true" data-attr-2="two"></i>
 				<span class="sr-only">Pencil icon</span>'
 			],
 			/** Font: icons array */