Once you've registered for VAT, your business is provided with a unique VAT number that other businesses need to reclaim the tax.
VAT or Value Added Tax is a percentage added to good and services you charge to consumers and businesses and only other businesses that are registered can reclaim from HMRC.
VAT numbers in the EU consist of 15 alphanumeric characters with the first two indicating the country of the registered business - e.g., GB for the UK. All the countries in the EU are currently as follows with their two letter country codes:
You can check if an EU or European number is valid on the EU site here.
The actual number is not a random set of numbers but a formula that can be checked to see if the number is valid. If you don't have a valid number, then your business can not re-claim the VAT.
For UK VAT numbers do the following exercise: Excluding the first two letters, list the numbers vertically and multiply each by a value starting with 8 and ending with 2. Then add up all the sums you have and deduct 97 from the sum until the answer is negative. The negative sum should be equal to the last 2 digits of the VAT number.
So for example, the VAT number for HSBC bank is GB 365684514 the calculation is:
The total of the above calculation is 24+42+30+30+32+12+10=180.
Deduct 97 twice from this to make the result negative and the result is 180-97-97=-14 which is the same as the last two digits so the VAT number is valid.