Question to the Department for Work and Pensions:

To ask Her Majesty's Government what characteristics of the National Insurance number system prevent them being aware how many National Insurance numbers are valid.

The National Insurance Number application process is robust, however there are instances where errors do occur. If it is identified that a National Insurance Number has been allocated and a record created in error, there are processes in place between the Department of Work and Pensions and HMRC to take action to prevent the National Insurance Number being used, for example in cases where a fraudulent application has been identified or a duplicate record has been created in error.