If you have registered with HMRC, you will have been issued a UTR number. Your UTR number does not change, once you are issued with a UTR number it will remain the same for your lifetime.
You will need a UTR number to submit a self-assessment tax return. There are a number of reasons why you may need to file a tax return and you can find the most common reasons here. It is important to know your UTR number before you submit your self assessment tax return. If you submit your tax return without your UTR number, it could result in a fine if HMRC are unable to match you to their records before the tax return deadline.
Read our FAQ here on where to find your UTR number.