diff --git a/src/Validator.php b/src/Validator.php
index bac22e5..12e7770 100644
--- a/src/Validator.php
+++ b/src/Validator.php
@@ -98,11 +98,10 @@ public function hasSupportedCountryPrefix(string $vatNumber): bool
      * Validate a VAT number format. This does not check whether the VAT number was really issued.
      *
      * @param string $vatNumber
-     * @param bool $skipIfUnsupported
      *
      * @return boolean
      */
-    public function validateVatNumberFormat(string $vatNumber, bool $skipIfUnsupported = false): bool
+    public function validateVatNumberFormat(string $vatNumber): bool
     {
         if ($vatNumber === '') {
             return false;
@@ -113,7 +112,7 @@ public function validateVatNumberFormat(string $vatNumber, bool $skipIfUnsupport
         $number = substr($vatNumber, 2);
 
         if (! isset($this->patterns[$country])) {
-            return $skipIfUnsupported;
+            throw new \InvalidArgumentException('The vat country prefix is not supported.');
         }
 
         return preg_match('/^' . $this->patterns[$country] . '$/', $number) > 0;
diff --git a/tests/ValidatorTest.php b/tests/ValidatorTest.php
index 847bc49..b4fb3f3 100644
--- a/tests/ValidatorTest.php
+++ b/tests/ValidatorTest.php
@@ -96,14 +96,6 @@ public function testValidateVatNumberFormat()
             'SI1234567',
             'SK123456789',
 
-            // valid number but with prefix
-            'invalid_prefix_GB999999973',
-            'invalid_prefix_IE1234567X',
-            'invalid_prefix_ESB1234567C',
-            'invalid_prefix_BE0123456789',
-            'invalid_prefix_MT12345678',
-            'invalid_prefix_LT123456789',
-
             // valid number but with suffix
             'IE1234567X_invalid_suffix',
             'ESB1234567C_invalid_suffix',