diff --git a/.phplint.yml b/.phplint.yml
index 35dda97..fd69190 100644
--- a/.phplint.yml
+++ b/.phplint.yml
@@ -2,3 +2,4 @@ path: ./
 exclude:
   - vendor
   - moodle/Tests/fixtures
+  - moodle/Tests/Sniffs/Namespaces/fixtures
diff --git a/moodle/Sniffs/Namespaces/NamespaceStatementSniff.php b/moodle/Sniffs/Namespaces/NamespaceStatementSniff.php
new file mode 100644
index 0000000..19923b3
--- /dev/null
+++ b/moodle/Sniffs/Namespaces/NamespaceStatementSniff.php
@@ -0,0 +1,71 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Checks that each file contains the standard GPL comment.
+ *
+ * @package    moodle-cs
+ * @copyright  2023 Andrew Lyons <andrew@nicols.co.uk>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+namespace MoodleHQ\MoodleCS\moodle\Sniffs\Namespaces;
+
+use PHP_CodeSniffer\Sniffs\Sniff;
+use PHP_CodeSniffer\Files\File;
+use PHP_CodeSniffer\Util\Tokens;
+
+// phpcs:disable moodle.NamingConventions
+
+class NamespaceStatementSniff implements Sniff {
+    public function register()
+    {
+        return [
+            T_NAMESPACE,
+        ];
+    }
+
+    public function process(File $file, $stackPtr)
+    {
+        $tokens = $file->getTokens();
+        // Format should be:
+        // - T_NAMESPACE
+        // - T_WHITESPACE
+        // - T_STRING
+
+        $checkPtr = $stackPtr + 2;
+        $token = $tokens[$checkPtr];
+        if ($token['code'] === T_NS_SEPARATOR) {
+            $fqdn = '';
+            $stop = $file->findNext(Tokens::$emptyTokens, ($stackPtr + 2));
+            for ($i = $stackPtr + 2; $i < $stop; $i++) {
+                $fqdn .= $tokens[$i]['content'];
+            }
+            $fix = $file->addFixableError(
+                'Namespace should not start with a slash: %s',
+                $checkPtr,
+                'LeadingSlash',
+                [$fqdn]
+            );
+
+            if ($fix) {
+                $file->fixer->beginChangeset();
+                $file->fixer->replaceToken($checkPtr, '');
+                $file->fixer->endChangeset();
+            }
+        }
+    }
+}
diff --git a/moodle/Tests/MoodleCSBaseTestCase.php b/moodle/Tests/MoodleCSBaseTestCase.php
index a2dfaaf..623e15c 100644
--- a/moodle/Tests/MoodleCSBaseTestCase.php
+++ b/moodle/Tests/MoodleCSBaseTestCase.php
@@ -80,53 +80,18 @@ public function set_component_mapping(array $mapping): void {
      *
      * @param string $standard name of the standard to be tested.
      */
-    protected function set_standard($standard) {
-        $installedStandards = \PHP_CodeSniffer\Util\Standards::getInstalledStandardDetails();
-
-        foreach (array_keys($installedStandards) as $standard) {
-            if (\PHP_CodeSniffer\Util\Standards::isInstalledStandard($standard) === false) {
-                // They didn't select a valid coding standard, so help them
-                // out by letting them know which standards are installed.
-                $error = 'ERROR: the "'.$standard.'" coding standard is not installed. ';
-                ob_start();
-                \PHP_CodeSniffer\Util\Standards::printInstalledStandards();
-                $error .= ob_get_contents();
-                ob_end_clean();
-                throw new \PHP_CodeSniffer\Exceptions\DeepExitException($error, 3);
-            }
+    protected function set_standard(string$standard) {
+        if (\PHP_CodeSniffer\Util\Standards::isInstalledStandard($standard) === false) {
+            // They didn't select a valid coding standard, so help them
+            // out by letting them know which standards are installed.
+            $error = "ERROR: the '{$standard}' coding standard is not installed.\n";
+            ob_start();
+            \PHP_CodeSniffer\Util\Standards::printInstalledStandards();
+            $error .= ob_get_contents();
+            ob_end_clean();
+            throw new \PHP_CodeSniffer\Exceptions\DeepExitException($error, 3);
         }
         $this->standard = $standard;
-        return;
-        // Since 2.9 arbitrary standard directories are not allowed by default,
-        // only those under the CodeSniffer/Standards dir are detected. Other base
-        // dirs containing standards can be added using CodeSniffer.conf or the
-        // PHP_CODESNIFFER_CONFIG_DATA global (installed_paths setting).
-        // We are using the global way here to avoid changes in the phpcs import.
-        // phpcs:disable
-        if (!isset($GLOBALS['PHP_CODESNIFFER_CONFIG_DATA']['installed_paths'])) {
-            $localcodecheckerpath = realpath(__DIR__ . '/../');
-            $GLOBALS['PHP_CODESNIFFER_CONFIG_DATA'] = ['installed_paths' => $localcodecheckerpath];
-        }
-        // phpcs:enable
-
-        // Basic search of standards in the allowed directories.
-        $stdsearch = array(
-            __DIR__ . '/../phpcs/src/Standards', // PHPCS standards dir.
-            __DIR__ . '/..',                     // Plugin local_codechecker dir, allowed above via global.
-        );
-
-        foreach ($stdsearch as $stdpath) {
-            $stdpath = realpath($stdpath . '/' . $standard);
-            $stdfile = $stdpath . '/ruleset.xml';
-            if (file_exists($stdfile)) {
-                $this->standard = $stdpath; // Need to pass the path here.
-                break;
-            }
-        }
-        // Standard not found, fail.
-        if ($this->standard === null) {
-            $this->fail('Standard "' . $standard . '" not found.');
-        }
     }
 
     /**
diff --git a/moodle/Tests/Sniffs/Namespaces/NamespaceStatementSniffTest.php b/moodle/Tests/Sniffs/Namespaces/NamespaceStatementSniffTest.php
new file mode 100644
index 0000000..d2a0d98
--- /dev/null
+++ b/moodle/Tests/Sniffs/Namespaces/NamespaceStatementSniffTest.php
@@ -0,0 +1,76 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+namespace MoodleHQ\MoodleCS\moodle\Tests\Sniffs\Namespaces;
+
+use MoodleHQ\MoodleCS\moodle\Tests\MoodleCSBaseTestCase;
+
+// phpcs:disable moodle.NamingConventions
+
+/**
+ * Test the NoLeadingSlash sniff.
+ *
+ * @package    moodle-cs
+ * @category   test
+ * @copyright  2023 Andrew Lyons <andrew@nicols.co.uk>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ *
+ * @covers \MoodleHQ\MoodleCS\moodle\Sniffs\Namespaces\NoLeadingSlashSniff
+ */
+class NamespaceStatementSniffTest extends MoodleCSBaseTestCase
+{
+    public static function leading_slash_provider(): array
+    {
+        return [
+            [
+                'fixture' => 'correct_namespace',
+                'warnings' => [],
+                'errors' => [],
+            ],
+            [
+                'fixture' => 'leading_backslash',
+                'warnings' => [],
+                'errors' => [
+                    3 => 'Namespace should not start with a slash: \MoodleHQ\MoodleCS\moodle\Tests\Sniffs\Namespaces',
+                ],
+            ],
+            [
+                'fixture' => 'curly_namespace',
+                'warnings' => [],
+                'errors' => [
+                    3 => 'Namespace should not start with a slash: \MoodleHQ\MoodleCS\moodle\Tests\Sniffs\Namespaces',
+                ],
+            ],
+        ];
+    }
+    /**
+     * @dataProvider leading_slash_provider
+     */
+    public function test_leading_slash(
+        string $fixture,
+        array $warnings,
+        array $errors
+    ): void
+    {
+        $this->set_standard('moodle');
+        $this->set_sniff('moodle.Namespaces.NamespaceStatement');
+        $this->set_fixture(sprintf("%s/fixtures/%s.php", __DIR__, $fixture));
+        $this->set_warnings($warnings);
+        $this->set_errors($errors);
+
+        $this->verify_cs_results();
+    }
+}
diff --git a/moodle/Tests/Sniffs/Namespaces/fixtures/correct_namespace.php b/moodle/Tests/Sniffs/Namespaces/fixtures/correct_namespace.php
new file mode 100644
index 0000000..26c4931
--- /dev/null
+++ b/moodle/Tests/Sniffs/Namespaces/fixtures/correct_namespace.php
@@ -0,0 +1,8 @@
+<?php
+
+namespace MoodleHQ\MoodleCS\moodle\Tests\Sniffs\Namespaces;
+
+defined('MOODLE_INTERNAL') || die(); // Make this always the 1st line in all CS fixtures.
+
+class correct_test extends base_test {
+}
diff --git a/moodle/Tests/Sniffs/Namespaces/fixtures/curly_namespace.php b/moodle/Tests/Sniffs/Namespaces/fixtures/curly_namespace.php
new file mode 100644
index 0000000..b5c4dbe
--- /dev/null
+++ b/moodle/Tests/Sniffs/Namespaces/fixtures/curly_namespace.php
@@ -0,0 +1,5 @@
+<?php
+
+namespace \MoodleHQ\MoodleCS\moodle\Tests\Sniffs\Namespaces {
+    class leading_backslash {}
+}
diff --git a/moodle/Tests/Sniffs/Namespaces/fixtures/curly_namespace.php.fixed b/moodle/Tests/Sniffs/Namespaces/fixtures/curly_namespace.php.fixed
new file mode 100644
index 0000000..97af8e3
--- /dev/null
+++ b/moodle/Tests/Sniffs/Namespaces/fixtures/curly_namespace.php.fixed
@@ -0,0 +1,5 @@
+<?php
+
+namespace MoodleHQ\MoodleCS\moodle\Tests\Sniffs\Namespaces {
+    class leading_backslash {}
+}
diff --git a/moodle/Tests/Sniffs/Namespaces/fixtures/curly_namespace_correct.php b/moodle/Tests/Sniffs/Namespaces/fixtures/curly_namespace_correct.php
new file mode 100644
index 0000000..97af8e3
--- /dev/null
+++ b/moodle/Tests/Sniffs/Namespaces/fixtures/curly_namespace_correct.php
@@ -0,0 +1,5 @@
+<?php
+
+namespace MoodleHQ\MoodleCS\moodle\Tests\Sniffs\Namespaces {
+    class leading_backslash {}
+}
diff --git a/moodle/Tests/Sniffs/Namespaces/fixtures/leading_backslash.php b/moodle/Tests/Sniffs/Namespaces/fixtures/leading_backslash.php
new file mode 100644
index 0000000..4f9471f
--- /dev/null
+++ b/moodle/Tests/Sniffs/Namespaces/fixtures/leading_backslash.php
@@ -0,0 +1,8 @@
+<?php
+
+namespace \MoodleHQ\MoodleCS\moodle\Tests\Sniffs\Namespaces;
+
+defined('MOODLE_INTERNAL') || die(); // Make this always the 1st line in all CS fixtures.
+
+class leading_backslash extends base_test {
+}
diff --git a/moodle/Tests/Sniffs/Namespaces/fixtures/leading_backslash.php.fixed b/moodle/Tests/Sniffs/Namespaces/fixtures/leading_backslash.php.fixed
new file mode 100644
index 0000000..a211a7f
--- /dev/null
+++ b/moodle/Tests/Sniffs/Namespaces/fixtures/leading_backslash.php.fixed
@@ -0,0 +1,8 @@
+<?php
+
+namespace MoodleHQ\MoodleCS\moodle\Tests\Sniffs\Namespaces;
+
+defined('MOODLE_INTERNAL') || die(); // Make this always the 1st line in all CS fixtures.
+
+class leading_backslash extends base_test {
+}
diff --git a/moodle/ruleset.xml b/moodle/ruleset.xml
index fbb1752..46cc140 100644
--- a/moodle/ruleset.xml
+++ b/moodle/ruleset.xml
@@ -107,6 +107,12 @@
         <severity>0</severity>
     </rule>
 
+    <!--
+        Namespace statements, and class imports (use statements) should not use a leading backslash.
+    -->
+    <rule ref="PSR12.Files.ImportStatement.LeadingSlash"/>
+    <rule ref="moodle.Namespaces.NamespaceStatement"/>
+
     <!-- Let's add the complete PHPCompatibility standard -->
     <rule ref="PHPCompatibility" />