Banks do need it when applying for some accounts in the UK, in particular ISAs because of the tax implications. I've also had it requested by a share broker. A

Banks do need it when applying for some accounts in the UK, in particular ISAs because of the tax implications. I've also had it requested by a share broker. A list of places that use it is here: https://www.gov.uk/national-insurance/your-national-insurance-number https://www.gov.uk/national-insurance/your-national-insuranc... Note the pages says: "To prevent identity fraud, keep your National Insurance number safe. Don’t share it with anyone who doesn’t need it." !