diff --git a/.gitignore b/.gitignore
index c7f0489..d76797f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,4 +2,5 @@ vendor/
 build/
 composer.lock
 .scannerwork/
+.idea/
 
diff --git a/composer.json b/composer.json
index 5337ce1..8cd3dd0 100644
--- a/composer.json
+++ b/composer.json
@@ -4,7 +4,6 @@
     "description": "EU VAT numbers validation using the VIES Service of the European Commission",
     "keywords": ["VIES","VAT","EU","EC"],
     "homepage": "https://github.com/DragonBe/vies",
-    "minimum-stability": "stable",
     "license": "MIT",
     "authors": [
         {
diff --git a/src/Vies/Validator/ValidatorXI.php b/src/Vies/Validator/ValidatorXI.php
new file mode 100644
index 0000000..3d27dea
--- /dev/null
+++ b/src/Vies/Validator/ValidatorXI.php
@@ -0,0 +1,111 @@
+<?php
+
+declare (strict_types=1);
+
+/**
+ * \DragonBe\Vies
+ *
+ * @author  Paweł Krzaczkowski <krzaczek+github@gmail.com>
+ * @license  MIT
+ */
+
+namespace DragonBe\Vies\Validator;
+
+/**
+ * This validator was derived from the "ValidatorGB" validator to validate the "XI" VAT numbers of
+ * "United Kingdom (Northern Ireland)" following the end of the Brexit transition period on December, 31 2020.
+ *
+ * It is based on the information at:
+ * https://www.avalara.com/vatlive/en/vat-news/brexit-northern-ireland-vat-and-eoro--xi--number.html
+ *
+ * As more information is published, this validator may be made more precise:
+ *
+ * - Remove the logic to validate Isle of Man numbers?
+ * - Remove the logic to validate 'GD' and 'HA' prefixes?
+ *
+ */
+
+/**
+ * Class ValidatorXI
+ * @package DragonBe\Vies\Validator
+ *
+ * VAT format: [C1 C2 C3 C4 C5]
+ *             [C1 C2 C3 C4 C5 C6 C7 C8 C9]
+ *             [C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12]
+ *
+ * Range:
+ *      C1 .. C2 Alphanumeric from A to Z or 0 to 9
+ *      C2 .. C11 Numeric from 0 to 9
+ *
+ * Rules:
+ * Case 1:
+ *      [C1 C2] Alpha: “GD” or “HA”
+ *      C3 ... C5 Numeric from 0 to 9
+ *
+ *      if [C1 C2] = “GD”
+ *          [C3 C4 C5] from 000 to 499
+ *
+ *      If [C1 C2] = “HA”
+ *          [C3 C4 C5] from 500 to 999
+ *
+ * Case 2
+ *      [C1 C2 C3] from 000 to 009 are numbers for Isle of Man
+ *      [C10 C11 C12] > 000
+ *      [C1 C2 C3 C4 C5 C6 C7 C8 C9] >000000000
+ *
+ *      [C8 C9]
+ *          R1 = (8*C1 + 7*C2 + 6*C3 + 5*C4 + 4*C5 + 3*C6 + 2*C7 + C8C9) modulo 97
+ *          R2 = ((8*C1 + 7*C2 + 6*C3 + 5*C4 + 4*C5 + 3*C6 + 2*C7 + C8C9) + 55) modulo 97
+ *          Either R1 or R2 must equal to zero.
+ *
+ */
+class ValidatorXI extends ValidatorAbstract
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function validate(string $vatNumber): bool
+    {
+        if (strlen($vatNumber) == 5) {
+            return $this->validateGovernment($vatNumber);
+        }
+
+        if (strlen($vatNumber) != 9 && strlen($vatNumber) != 12) {
+            return false;
+        }
+
+        $weights = [8, 7, 6, 5, 4, 3, 2];
+        $checkVal = $this->sumWeights($weights, $vatNumber);
+        $checkVal += (int)substr($vatNumber, 7, 2);
+
+        $Result1 = $checkVal % 97;
+        $Result2 = ($Result1 + 55) % 97;
+
+        return ! ($Result1 * $Result2);
+    }
+
+    /**
+     * Validate Government VAT
+     *
+     * @param  string $vatNumber
+     *
+     * @return bool
+     */
+    private function validateGovernment(string $vatNumber): bool
+    {
+        $prefix = strtoupper(substr($vatNumber, 0, 2));
+        $number = (int) substr($vatNumber, 2, 3);
+
+        // Government departments
+        if ($prefix == 'GD') {
+            return $number < 500;
+        }
+
+        // Health authorities
+        if ($prefix == 'HA') {
+            return $number > 499;
+        }
+
+        return false;
+    }
+}
diff --git a/src/Vies/Vies.php b/src/Vies/Vies.php
index 3e751e6..1c8509e 100644
--- a/src/Vies/Vies.php
+++ b/src/Vies/Vies.php
@@ -51,7 +51,7 @@ class Vies
     const VIES_PORT = 80;
     const VIES_WSDL = '/taxation_customs/vies/checkVatService.wsdl';
     const VIES_TEST_WSDL = '/taxation_customs/vies/checkVatTestService.wsdl';
-    const VIES_EU_COUNTRY_TOTAL = 28;
+    const VIES_EU_COUNTRY_TOTAL = 29;
     const VIES_TEST_VAT_NRS = [100, 200, 201, 202, 300, 301, 302, 400, 401, 500, 501, 600, 601];
 
     protected const VIES_EU_COUNTRY_LIST = [
@@ -83,6 +83,7 @@ class Vies
         'SI' => ['name' => 'Slovenia', 'validator' => Validator\ValidatorSI::class],
         'SK' => ['name' => 'Slovakia', 'validator' => Validator\ValidatorSK::class],
         'GB' => ['name' => 'United Kingdom', 'validator' => Validator\ValidatorGB::class],
+        'XI' => ['name' => 'United Kingdom (Northern Ireland)', 'validator' => Validator\ValidatorXI::class],
         'EU' => ['name' => 'MOSS Number', 'validator' => Validator\ValidatorEU::class],
     ];
 
diff --git a/tests/Vies/Validator/ValidatorXITest.php b/tests/Vies/Validator/ValidatorXITest.php
new file mode 100644
index 0000000..1276ee4
--- /dev/null
+++ b/tests/Vies/Validator/ValidatorXITest.php
@@ -0,0 +1,31 @@
+<?php
+
+declare (strict_types=1);
+
+namespace DragonBe\Test\Vies\Validator;
+
+class ValidatorXITest extends AbstractValidatorTest
+{
+    /**
+     * @covers \DragonBe\Vies\Validator\ValidatorXI
+     * @dataProvider vatNumberProvider
+     */
+    public function testValidator(string $vatNumber, bool $state)
+    {
+        $this->validateVatNumber('XI', $vatNumber, $state);
+    }
+
+    public function vatNumberProvider()
+    {
+        return [
+            ['925901618', true],
+            ['GD001', true],
+            ['HA500', true],
+            ['434031493', false],
+            ['12345', false],
+            ['GD500', false],
+            ['HA100', false],
+            ['12345678', false],
+        ];
+    }
+}