diff --git a/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php b/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php
index 6d68c1e4bc20affae374141e86e8a0f1bdcda8f1..2569ee4adbaba186634f2a542674a1cd40806c9e 100644
--- a/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php
+++ b/core/lib/Drupal/Core/Utility/UnroutedUrlAssembler.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\Core\Utility;
 
+use Drupal\Component\Utility\NestedArray;
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\Core\GeneratedUrl;
 use Drupal\Core\PathProcessor\OutboundPathProcessorInterface;
@@ -69,13 +70,18 @@ public function assemble($uri, array $options = [], $collect_bubbleable_metadata
    */
   protected function buildExternalUrl($uri, array $options = [], $collect_bubbleable_metadata = FALSE) {
     $this->addOptionDefaults($options);
-    // Split off the fragment.
-    if (strpos($uri, '#') !== FALSE) {
-      list($uri, $old_fragment) = explode('#', $uri, 2);
-      // If $options contains no fragment, take it over from the path.
-      if (isset($old_fragment) && !$options['fragment']) {
-        $options['fragment'] = '#' . $old_fragment;
-      }
+    // Split off the query & fragment.
+    $parsed = UrlHelper::parse($uri);
+    $uri = $parsed['path'];
+
+    $parsed += ['query' => []];
+    $options += ['query' => []];
+
+    $options['query'] = NestedArray::mergeDeep($parsed['query'], $options['query']);
+    ksort($options['query']);
+
+    if ($parsed['fragment'] && !$options['fragment']) {
+      $options['fragment'] = '#' . $parsed['fragment'];
     }
 
     if (isset($options['https'])) {
@@ -88,7 +94,7 @@ protected function buildExternalUrl($uri, array $options = [], $collect_bubbleab
     }
     // Append the query.
     if ($options['query']) {
-      $uri .= (strpos($uri, '?') !== FALSE ? '&' : '?') . UrlHelper::buildQuery($options['query']);
+      $uri .= '?' . UrlHelper::buildQuery($options['query']);
     }
     // Reassemble.
     $url = $uri . $options['fragment'];
diff --git a/core/modules/rdf/tests/src/Kernel/Field/LinkFieldRdfaTest.php b/core/modules/rdf/tests/src/Kernel/Field/LinkFieldRdfaTest.php
index 30735e3a8b6fd358a18e473b8893fdd8a29f4107..aac9b75591f861b4d4265f8dad834f99214ebfcf 100644
--- a/core/modules/rdf/tests/src/Kernel/Field/LinkFieldRdfaTest.php
+++ b/core/modules/rdf/tests/src/Kernel/Field/LinkFieldRdfaTest.php
@@ -42,7 +42,7 @@ protected function setUp() {
    */
   public function testAllFormattersExternal() {
     // Set up test values.
-    $this->testValue = 'http://test.me/foo/bar/neque/porro/quisquam/est/qui-dolorem?foo/bar/neque/porro/quisquam/est/qui-dolorem';
+    $this->testValue = 'http://test.me/foo/bar/neque/porro/quisquam/est/qui-dolorem?path=foo/bar/neque/porro/quisquam/est/qui-dolorem';
     $this->entity = EntityTest::create([]);
     $this->entity->{$this->fieldName}->uri = $this->testValue;
 
diff --git a/core/modules/system/tests/src/Functional/Common/UrlTest.php b/core/modules/system/tests/src/Functional/Common/UrlTest.php
index cd27ca79a5d9caf1c540e01b22979f09ececd665..8e4d7b121c5852a7d7ca65ebf85d2350f7734d9e 100644
--- a/core/modules/system/tests/src/Functional/Common/UrlTest.php
+++ b/core/modules/system/tests/src/Functional/Common/UrlTest.php
@@ -306,15 +306,15 @@ public function testExternalUrls() {
 
     // Verify external URL can be extended with a query string.
     $url = $test_url;
-    $query = [$this->randomMachineName(5) => $this->randomMachineName(5)];
+    $query = ['awesome' => 'drupal'];
     $result = Url::fromUri($url, ['query' => $query])->toString();
-    $this->assertEqual($url . '?' . http_build_query($query, '', '&'), $result, 'External URL can be extended with a query string in $options.');
+    $this->assertSame('https://www.drupal.org/?awesome=drupal', $result);
 
     // Verify query string can be extended in an external URL.
     $url = $test_url . '?drupal=awesome';
-    $query = [$this->randomMachineName(5) => $this->randomMachineName(5)];
+    $query = ['awesome' => 'drupal'];
     $result = Url::fromUri($url, ['query' => $query])->toString();
-    $this->assertEqual($url . '&' . http_build_query($query, '', '&'), $result);
+    $this->assertEqual('https://www.drupal.org/?awesome=drupal&drupal=awesome', $result);
   }
 
 }
diff --git a/core/tests/Drupal/Tests/Core/Utility/UnroutedUrlAssemblerTest.php b/core/tests/Drupal/Tests/Core/Utility/UnroutedUrlAssemblerTest.php
index a47d2062ea76c4f39e53aa6757ae91ac7ae1cb28..70b3880f4c1df8c7eebf4d5d3b1a6ddbb48ba34c 100644
--- a/core/tests/Drupal/Tests/Core/Utility/UnroutedUrlAssemblerTest.php
+++ b/core/tests/Drupal/Tests/Core/Utility/UnroutedUrlAssemblerTest.php
@@ -96,6 +96,10 @@ public function providerTestAssembleWithExternalUrl() {
       ['http://example.com/test', ['https' => TRUE], 'https://example.com/test'],
       ['https://example.com/test', ['https' => FALSE], 'http://example.com/test'],
       ['https://example.com/test?foo=1#bar', [], 'https://example.com/test?foo=1#bar'],
+      'override-query' => ['https://example.com/test?foo=1#bar', ['query' => ['foo' => 2]], 'https://example.com/test?foo=2#bar'],
+      'override-query-merge' => ['https://example.com/test?foo=1#bar', ['query' => ['bar' => 2]], 'https://example.com/test?bar=2&foo=1#bar'],
+      'override-deep-query-merge' => ['https://example.com/test?foo=1#bar', ['query' => ['bar' => ['baz' => 'foo']]], 'https://example.com/test?bar%5Bbaz%5D=foo&foo=1#bar'],
+      'override-fragment' => ['https://example.com/test?foo=1#bar', ['fragment' => 'baz'], 'https://example.com/test?foo=1#baz'],
       ['//www.drupal.org', [], '//www.drupal.org'],
     ];
   }