Why would a UK bank need your National Insurance number when you want to open a savings account?

I don't mean an ISA, just a fixed-term deposit account. Far as I can see they have no need for that detail.
Why would a UK bank need your National Insurance number when you want to open a savings account?
Because they have to pay tax on the interest that you earn and they need your NI number for tax purposes.





-EDIT- If they don't pay tax, they will need to complete an R85 (Tax Exemption Form), which also requires their NI number. Again, it's all tax related, but the bank will definately need to know their NI number, it's a regulation.