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Country code GB followed by either:
standard: 9 digits (block of 3, block of 4, block of 2 – e.g. GB9999999 73)
branch traders: 12 digits (as for 9 digits, followed by a block of 3 digits)
government departments: the letters GD then 3 digits from 000 to 499 (e.g. GBGD001)
health authorities: the letters HA then 3 digits from 500 to 999 (e.g. GBHA599)
For the 9-digit scheme, the 2-digit block containing the 8th and 9th digitsis always in the range 00 to 96 and is derived from a weighted modulus-97 check number (an identical algorithm is used for the 12-digit scheme, ignoring the extra 3-digit block).The current modulus-97 series is expected to run out during 2010, so a parallel series of numbers is being introduced from November 2009 for new registrations, restarting at 100 nnnn nn and following the same format but with the last two digits derived from an alternative algorithm known as “9755″. The algorithm is identical to the one for the established series except that 55 is subtracted to give thecheck number (modulus 97), so the check number is either 55 less than or (if this would be negative) 42 greater than the check number that a VAT number in the established series would have if it were identical in the first seven digits. The details of the 97−55 check algorithm were to be secret but are now available from HMRC on request.
The GD and HA formats may also be formatted as GB888 8xxx yy for EU compatibility, where xxx is the 3-digit number from the short format and yy is the2-digit modulus-97 check number.
Isle of Man registrations share the 9- and 12-digit formats with the UK, with GB as the country code prefix, but are distinguished by having 00 as thefirst two digits. Numbers with 01 to 09 in the first two digits are reserved by HM Revenue & Customs for UK non-VAT reference schemes.
source: http://vat-number-validation.eu/
standard: 9 digits (block of 3, block of 4, block of 2 – e.g. GB9999999 73)
branch traders: 12 digits (as for 9 digits, followed by a block of 3 digits)
government departments: the letters GD then 3 digits from 000 to 499 (e.g. GBGD001)
health authorities: the letters HA then 3 digits from 500 to 999 (e.g. GBHA599)
For the 9-digit scheme, the 2-digit block containing the 8th and 9th digitsis always in the range 00 to 96 and is derived from a weighted modulus-97 check number (an identical algorithm is used for the 12-digit scheme, ignoring the extra 3-digit block).The current modulus-97 series is expected to run out during 2010, so a parallel series of numbers is being introduced from November 2009 for new registrations, restarting at 100 nnnn nn and following the same format but with the last two digits derived from an alternative algorithm known as “9755″. The algorithm is identical to the one for the established series except that 55 is subtracted to give thecheck number (modulus 97), so the check number is either 55 less than or (if this would be negative) 42 greater than the check number that a VAT number in the established series would have if it were identical in the first seven digits. The details of the 97−55 check algorithm were to be secret but are now available from HMRC on request.
The GD and HA formats may also be formatted as GB888 8xxx yy for EU compatibility, where xxx is the 3-digit number from the short format and yy is the2-digit modulus-97 check number.
Isle of Man registrations share the 9- and 12-digit formats with the UK, with GB as the country code prefix, but are distinguished by having 00 as thefirst two digits. Numbers with 01 to 09 in the first two digits are reserved by HM Revenue & Customs for UK non-VAT reference schemes.
source: http://vat-number-validation.eu/