diff --git a/RELEASE-NOTES-1.40 b/RELEASE-NOTES-1.40
index 5f052573be1..da37cff73a1 100644
--- a/RELEASE-NOTES-1.40
+++ b/RELEASE-NOTES-1.40
@@ -62,6 +62,8 @@ For notes on 1.39.x and older releases, see HISTORY.
 * (T277618) The @noVarDump annotation from the DebugInfoTrait tool can now be
   added to references to stop them from being expanded when their object is
   passed to var_dump(), to make its use for debugging more feasible.
+* Added 'GetBlockErrorMessageKey' hook, allow extensions'
+  block error messages to be received and displayed by BlockErrorFormatter.
 * …
 
 === External library changes in 1.40 ===
diff --git a/includes/Hook/GetBlockErrorMessageKey.php b/includes/Hook/GetBlockErrorMessageKey.php
new file mode 100644
index 00000000000..182c0d00eb0
--- /dev/null
+++ b/includes/Hook/GetBlockErrorMessageKey.php
@@ -0,0 +1,27 @@
+<?php
+
+namespace MediaWiki\Hook;
+
+use MediaWiki\Block\Block;
+
+/**
+ * This is a hook handler interface, see docs/Hooks.md.
+ * Use the hook name "GetBlockErrorMessageKey" to register handlers implementing this interface.
+ *
+ * @stable to implement
+ * @ingroup Hooks
+ */
+interface GetBlockErrorMessageKey {
+	/**
+	 * This hook is called in BlockErrorFormatter to allow
+	 * extensions to override the message that will be displayed
+	 * to the user.
+	 *
+	 * @since 1.40
+	 *
+	 * @param Block $block
+	 * @param string &$key
+	 * @return bool|void True or no return value to continue or false to abort
+	 */
+	public function onGetBlockErrorMessageKey( $block, &$key );
+}
diff --git a/includes/HookContainer/HookRunner.php b/includes/HookContainer/HookRunner.php
index 8fc68151419..301d05d9670 100644
--- a/includes/HookContainer/HookRunner.php
+++ b/includes/HookContainer/HookRunner.php
@@ -182,6 +182,7 @@ class HookRunner implements
 	\MediaWiki\Hook\FileUploadHook,
 	\MediaWiki\Hook\FormatAutocommentsHook,
 	\MediaWiki\Hook\GalleryGetModesHook,
+	\MediaWiki\Hook\GetBlockErrorMessageKey,
 	\MediaWiki\Hook\GetCacheVaryCookiesHook,
 	\MediaWiki\Hook\GetCanonicalURLHook,
 	\MediaWiki\Hook\GetDefaultSortkeyHook,
@@ -2589,6 +2590,13 @@ class HookRunner implements
 		);
 	}
 
+	public function onGetBlockErrorMessageKey( $block, &$key ) {
+		return $this->container->run(
+			'GetBlockErrorMessageKey',
+			[ $block, &$key ]
+		);
+	}
+
 	public function onModifyExportQuery( $db, &$tables, $cond, &$opts,
 		&$join_conds, &$conds
 	) {
diff --git a/includes/ServiceWiring.php b/includes/ServiceWiring.php
index c4a45623a05..66eb6439b57 100644
--- a/includes/ServiceWiring.php
+++ b/includes/ServiceWiring.php
@@ -297,7 +297,8 @@ return [
 
 	'BlockErrorFormatter' => static function ( MediaWikiServices $services ): BlockErrorFormatter {
 		return new BlockErrorFormatter(
-			$services->getTitleFormatter()
+			$services->getTitleFormatter(),
+			$services->getHookContainer()
 		);
 	},
 
diff --git a/includes/block/BlockErrorFormatter.php b/includes/block/BlockErrorFormatter.php
index 9c70508cb80..4726cc82f41 100644
--- a/includes/block/BlockErrorFormatter.php
+++ b/includes/block/BlockErrorFormatter.php
@@ -22,6 +22,8 @@ namespace MediaWiki\Block;
 
 use CommentStoreComment;
 use Language;
+use MediaWiki\HookContainer\HookContainer;
+use MediaWiki\HookContainer\HookRunner;
 use MediaWiki\Page\PageReferenceValue;
 use MediaWiki\User\UserIdentity;
 use Message;
@@ -38,13 +40,19 @@ class BlockErrorFormatter {
 	/** @var TitleFormatter */
 	private $titleFormatter;
 
+	/** @var HookRunner */
+	private $hookRunner;
+
 	/**
 	 * @param TitleFormatter $titleFormatter
+	 * @param HookContainer $hookContainer
 	 */
 	public function __construct(
-		TitleFormatter $titleFormatter
+		TitleFormatter $titleFormatter,
+		HookContainer $hookContainer
 	) {
 		$this->titleFormatter = $titleFormatter;
+		$this->hookRunner = new HookRunner( $hookContainer );
 	}
 
 	/**
@@ -186,6 +194,10 @@ class BlockErrorFormatter {
 		} elseif ( $block instanceof CompositeBlock ) {
 			$key = 'blockedtext-composite';
 		}
+
+		// runs a hook that allows extensions to check block characteristics
+		$this->hookRunner->onGetBlockErrorMessageKey( $block, $key );
+
 		return $key;
 	}