diff --git a/UPGRADE-3.3.md b/UPGRADE-3.3.md
index 036ad1f61cf7c..5e83a3bf3a974 100644
--- a/UPGRADE-3.3.md
+++ b/UPGRADE-3.3.md
@@ -252,8 +252,12 @@ Workflow
 Yaml
 ----
 
- * Deprecated support for implicitly parsing non-string mapping keys as strings. Mapping keys that are no strings will
-   lead to a `ParseException` in Symfony 4.0. Use the `PARSE_KEYS_AS_STRINGS` flag to opt-in for keys to be parsed as
+ * Starting an unquoted string with a question mark followed by a space is
+   deprecated and will throw a `ParseException` in Symfony 4.0.
+
+ * Deprecated support for implicitly parsing non-string mapping keys as strings.
+   Mapping keys that are no strings will lead to a `ParseException` in Symfony
+   4.0. Use the `PARSE_KEYS_AS_STRINGS` flag to opt-in for keys to be parsed as
    strings.
 
    Before:
diff --git a/UPGRADE-4.0.md b/UPGRADE-4.0.md
index 1123d9a87a869..fd30de43aa726 100644
--- a/UPGRADE-4.0.md
+++ b/UPGRADE-4.0.md
@@ -463,8 +463,12 @@ Workflow
 Yaml
 ----
 
- * Removed support for implicitly parsing non-string mapping keys as strings. Mapping keys that are no strings will
-   result in a `ParseException`. Use the `PARSE_KEYS_AS_STRINGS` flag to opt-in for keys to be parsed as strings.
+ * Starting an unquoted string with a question mark followed by a space
+   throws a `ParseException`.
+
+ * Removed support for implicitly parsing non-string mapping keys as strings.
+   Mapping keys that are no strings will result in a `ParseException`. Use the
+   `PARSE_KEYS_AS_STRINGS` flag to opt-in for keys to be parsed as strings.
 
    Before:
 
diff --git a/src/Symfony/Component/Yaml/CHANGELOG.md b/src/Symfony/Component/Yaml/CHANGELOG.md
index fb62848470cf2..b50a7b875f337 100644
--- a/src/Symfony/Component/Yaml/CHANGELOG.md
+++ b/src/Symfony/Component/Yaml/CHANGELOG.md
@@ -4,8 +4,12 @@ CHANGELOG
 3.3.0
 -----
 
- * Deprecated support for implicitly parsing non-string mapping keys as strings. Mapping keys that are no strings will
-   lead to a `ParseException` in Symfony 4.0. Use the `PARSE_KEYS_AS_STRINGS` flag to opt-in for keys to be parsed as
+ * Starting an unquoted string with a question mark followed by a space is
+   deprecated and will throw a `ParseException` in Symfony 4.0.
+
+ * Deprecated support for implicitly parsing non-string mapping keys as strings.
+   Mapping keys that are no strings will lead to a `ParseException` in Symfony
+   4.0. Use the `PARSE_KEYS_AS_STRINGS` flag to opt-in for keys to be parsed as
    strings.
 
    Before:
diff --git a/src/Symfony/Component/Yaml/Parser.php b/src/Symfony/Component/Yaml/Parser.php
index 40ad7244e77d0..867244163313a 100644
--- a/src/Symfony/Component/Yaml/Parser.php
+++ b/src/Symfony/Component/Yaml/Parser.php
@@ -144,6 +144,10 @@ public function parse($value, $flags = 0)
                     $values['value'] = $matches['value'];
                 }
 
+                if (isset($values['value'][1]) && '?' === $values['value'][0] && ' ' === $values['value'][1]) {
+                    @trigger_error('Starting an unquoted string with a question mark followed by a space is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0.', E_USER_DEPRECATED);
+                }
+
                 // array
                 if (!isset($values['value']) || '' == trim($values['value'], ' ') || 0 === strpos(ltrim($values['value'], ' '), '#')) {
                     $data[] = $this->parseBlock($this->getRealCurrentLineNb() + 1, $this->getNextEmbedBlock(null, true), $flags);
@@ -301,6 +305,10 @@ public function parse($value, $flags = 0)
                     throw new ParseException('Multiple documents are not supported.', $this->currentLineNb + 1, $this->currentLine);
                 }
 
+                if (isset($this->currentLine[1]) && '?' === $this->currentLine[0] && ' ' === $this->currentLine[1]) {
+                    @trigger_error('Starting an unquoted string with a question mark followed by a space is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0.', E_USER_DEPRECATED);
+                }
+
                 // 1-liner optionally followed by newline(s)
                 if (is_string($value) && $this->lines[0] === trim($value)) {
                     try {
diff --git a/src/Symfony/Component/Yaml/Tests/ParserTest.php b/src/Symfony/Component/Yaml/Tests/ParserTest.php
index 01b17a147dbf4..dfbc989f98659 100644
--- a/src/Symfony/Component/Yaml/Tests/ParserTest.php
+++ b/src/Symfony/Component/Yaml/Tests/ParserTest.php
@@ -1640,6 +1640,52 @@ public function testExceptionWhenUsingUnsuportedBuiltInTags()
         $this->parser->parse('!!foo');
     }
 
+    /**
+     * @group legacy
+     * @expectedDeprecation Starting an unquoted string with a question mark followed by a space is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0.
+     */
+    public function testComplexMappingThrowsParseException()
+    {
+        $yaml = <<<YAML
+? "1"
+:
+  name: végétalien
+YAML;
+
+        $this->parser->parse($yaml);
+    }
+
+    /**
+     * @group legacy
+     * @expectedDeprecation Starting an unquoted string with a question mark followed by a space is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0.
+     */
+    public function testComplexMappingNestedInMappingThrowsParseException()
+    {
+        $yaml = <<<YAML
+diet:
+  ? "1"
+  :
+    name: végétalien
+YAML;
+
+        $this->parser->parse($yaml);
+    }
+
+    /**
+     * @group legacy
+     * @expectedDeprecation Starting an unquoted string with a question mark followed by a space is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0.
+     */
+    public function testComplexMappingNestedInSequenceThrowsParseException()
+    {
+        $yaml = <<<YAML
+- ? "1"
+  :
+    name: végétalien
+YAML;
+
+        $this->parser->parse($yaml);
+    }
+
     private function loadTestsFromFixtureFiles($testsFile)
     {
         $parser = new Parser();