If you are self-employed or fill out your own tax return you are probably aware of the need to pay your tax by 31 January. But you may also need to make another payment by 31 July, or you could have to pay an interest charge to HM Revenue & Customs (HMRC).
Payments on account spread the cost of your tax bill into two instalments over the year. The first payment on account is due by midnight on 31 January and is calculated by looking at your tax bill for the previous year, and the second is due by 31 July. Normally, both of those payments should add up to your tax bill for the previous year and they are assumed to be the same year on year.
If your income has fluctuated since your last tax bill you may owe a different amount of tax. If your income fell substantially you can request to have your payments on account reduced either by logging into your online account or sending form SA303 to the tax office.