A unique tax reference (UTR) is assigned to each anyone who registers for self assessment tax. It links them to their tax records at HMRC. Once your UTR is assigned, it stays with you forever.

Each UTR has 10 digits, a few of them end in a 'K'. It will appear on letters from HMRC, sometimes titled: Our Ref or Tax Ref.

If you've not yet got a UTR and have earned over the trading allowance, head to the Tax tab and tap Link your HMRC account. From there, you can register via Earnr.