A reference number used by the government is called the National Insurance number. The National Insurance number can be tracked in an official document such as the pay-slip, National insurance card, or letter from the HMRC / Department of work and pensions.
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'You LIED!' Jon Snow SNAPS at Tory minister over £240 National Insurance hike - https://newsexplored.co.uk/you-lied-jon-snow-snaps-at-tory-minister-over-240-national-insurance-hike/
Labour considers raising national insurance coverage to fix £30bn NHS 'black hole' - http://www.healtherpeople.com/labour-considers-raising-national-insurance-coverage-to-fix-30bn-nhs-black-hole.html
Tim Farron brands Budget an ‘omNICshambles’ after Tories break National Insurance promise - https://newsexplored.co.uk/tim-farron-brands-budget-an-omnicshambles-after-tories-break-national-insurance-promise/
Hammond Budget National Insurance rise due to 'new challenges' - BBC News
Budget 2017: National Insurance hikes for self-employed
Ministry of National Insurance poster - opens new window