diff --git a/src/Form/UpdaterForm.php b/src/Form/UpdaterForm.php
index 7a8e8250c8d09ff5f8205c1326d537ee7351e885..63e50ad978e61645adeb153d34593da7dc1ad34f 100644
--- a/src/Form/UpdaterForm.php
+++ b/src/Form/UpdaterForm.php
@@ -18,6 +18,7 @@ use Drupal\Core\State\StateInterface;
 use Drupal\Core\Url;
 use Drupal\package_manager\Exception\StageException;
 use Drupal\package_manager\Exception\StageOwnershipException;
+use Drupal\package_manager\ValidationResult;
 use Drupal\system\SystemManager;
 use Drupal\update\UpdateManagerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
@@ -31,7 +32,9 @@ use Symfony\Component\EventDispatcher\EventDispatcherInterface;
  */
 class UpdaterForm extends FormBase {
 
-  use ReadinessTrait;
+  use ReadinessTrait {
+    formatResult as traitFormatResult;
+  }
 
   /**
    * The updater service.
@@ -322,4 +325,20 @@ class UpdaterForm extends FormBase {
     batch_set($batch);
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function formatResult(ValidationResult $result) {
+    $messages = $result->getMessages();
+
+    if (count($messages) > 1) {
+      return [
+        '#theme' => 'item_list__automatic_updates_validation_results',
+        '#prefix' => $result->getSummary(),
+        '#items' => $messages,
+      ];
+    }
+    return $this->traitFormatResult($result);
+  }
+
 }
diff --git a/src/Validation/ReadinessTrait.php b/src/Validation/ReadinessTrait.php
index 12b0df150f66de4379b85e93b9e2622c4a355e29..a01906bd5450e2f3b82b4aac7fd4e6c37e87192d 100644
--- a/src/Validation/ReadinessTrait.php
+++ b/src/Validation/ReadinessTrait.php
@@ -5,6 +5,7 @@ namespace Drupal\automatic_updates\Validation;
 use Drupal\Core\Messenger\MessengerInterface;
 use Drupal\Core\Render\RendererInterface;
 use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\package_manager\ValidationResult;
 use Drupal\system\SystemManager;
 
 /**
@@ -79,13 +80,8 @@ trait ReadinessTrait {
     $build = [
       '#theme' => 'item_list__automatic_updates_validation_results',
       '#prefix' => $this->getFailureMessageForSeverity($severity),
+      '#items' => array_map([$this, 'formatResult'], $results),
     ];
-    foreach ($results as $result) {
-      $messages = $result->getMessages();
-      // @todo Find a way to show all the messages, not just the summary, if
-      //   we're on the updater form in https://drupal.org/i/3284346.
-      $build['#items'][] = count($messages) === 1 ? reset($messages) : $result->getSummary();
-    }
     $message = $renderer->renderRoot($build);
 
     if ($severity === SystemManager::REQUIREMENT_ERROR) {
@@ -96,4 +92,19 @@ trait ReadinessTrait {
     }
   }
 
+  /**
+   * Formats a single validation result as an item in an item list.
+   *
+   * @param \Drupal\package_manager\ValidationResult $result
+   *   A validation result.
+   *
+   * @return \Drupal\Core\StringTranslation\TranslatableMarkup|array
+   *   The validation result, formatted for inclusion in a themed item list as
+   *   either a translated string, or a renderable array.
+   */
+  protected function formatResult(ValidationResult $result) {
+    $messages = $result->getMessages();
+    return count($messages) === 1 ? reset($messages) : $result->getSummary();
+  }
+
 }
diff --git a/tests/src/Functional/UpdaterFormTest.php b/tests/src/Functional/UpdaterFormTest.php
index cd1ec9b1299685c7dcb32e427086470f288df05f..fd4a157606710828a883e092c26a1de277e1fdc4 100644
--- a/tests/src/Functional/UpdaterFormTest.php
+++ b/tests/src/Functional/UpdaterFormTest.php
@@ -168,9 +168,10 @@ class UpdaterFormTest extends AutomaticUpdatesFunctionalTestBase {
     $this->setCoreVersion('9.8.0');
     $this->checkForUpdates();
 
-    // Set up a new fake error.
+    // Set up a new fake error. Use an error with multiple messages so we can
+    // ensure that they're all displayed, along with their summary.
     $this->createTestValidationResults();
-    $expected_results = $this->testResults['checker_1']['1 error'];
+    $expected_results = [$this->testResults['checker_1']['2 errors 2 warnings']['1:errors']];
     TestSubscriber1::setTestResult($expected_results, ReadinessCheckEvent::class);
 
     // If a validator raises an error during readiness checking, the form should
@@ -182,6 +183,8 @@ class UpdaterFormTest extends AutomaticUpdatesFunctionalTestBase {
     // during the form build, which means the new error should be cached and
     // displayed instead of the previously cached error.
     $assert_session->pageTextContainsOnce((string) $expected_results[0]->getMessages()[0]);
+    $assert_session->pageTextContainsOnce((string) $expected_results[0]->getMessages()[1]);
+    $assert_session->pageTextContainsOnce((string) $expected_results[0]->getSummary());
     $assert_session->pageTextContainsOnce(static::$errorsExplanation);
     $assert_session->pageTextNotContains(static::$warningsExplanation);
     $assert_session->pageTextNotContains($cached_message);
@@ -218,9 +221,9 @@ class UpdaterFormTest extends AutomaticUpdatesFunctionalTestBase {
     $this->assertUpdateStagedTimes(0);
     $assert_session->pageTextContainsOnce('An error has occurred.');
     $page->clickLink('the error page');
-    // Since there's only one message, we shouldn't see the summary.
-    $assert_session->pageTextNotContains($expected_results[0]->getSummary());
+    $assert_session->pageTextContainsOnce($expected_results[0]->getSummary());
     $assert_session->pageTextContainsOnce((string) $expected_results[0]->getMessages()[0]);
+    $assert_session->pageTextContainsOnce((string) $expected_results[0]->getMessages()[1]);
     $assert_session->pageTextNotContains($cached_message);
   }