Northern Ireland VAT Number Validation

Northern Ireland VAT numbers use the XI prefix introduced after Brexit to identify businesses in Northern Ireland that trade with the EU under the Northern Ireland Protocol.

Format Overview

Regex Pattern

^XI([0-9]{9}|[0-9]{12}|GD[0-9]{3}|HA[0-9]{3})$

When is XI Used?

• Post-Brexit identifier for Northern Ireland businesses
• Used for intra-EU trade under the Northern Ireland Protocol
• Businesses may have both GB and XI VAT numbers

Checksum Algorithm (MOD 97 - GB variant)

XI uses the same validation as GB VAT numbers:

def validate_xi_vat(number):
    if number.startswith('XI'):
        number = number[2:]

    if number.startswith(("GD", "HA")):
        return True  # accepted special XI formats

    if len(number) not in (9, 12) or not number.isdigit():
        return False

    if len(number) == 12:
        # First 9 digits are the main number, last 3 are branch
        number = number[:9]

    weights = [8, 7, 6, 5, 4, 3, 2]
    total = sum(int(d) * w for d, w in zip(number[:7], weights))

    # Check with standard offset
    if total % 97 == 0:
        return True

    # Check with 55 offset (alternative valid format)
    if (total + 55) % 97 == 0:
        return True

    return False