National Insurance number
The National Insurance number is used as a reference number in the Pay As You Earn system, and also by the self-employed. It is also used in applications for Individual Savings Accounts (ISAs), to check that an individual has opened only one ISA in a tax year.
Tax policy
National Insurance in the United Kingdom and Social Security in the United States are forms of social welfare funded outside their national income tax systems, paid for through worker contributions, something labeled a stealth tax by critics.