National Insurance number

The National Insurance number is a number used in the United Kingdom, Guernsey, the Isle of Man and Jersey in the administration of the National Insurance or social security system. It is also used as a de facto national identification number in the UK, including in the tax system, banking, social welfare, online government services and electoral registration, despite it not being explicitly defined as such.

Source: Wikipedia — National Insurance number (CC BY-SA 4.0)