Most of the nine million taxpayers who file a self-assessment return must make a payment by 31 July.
If you forget or can't make a payment by the deadline you face interest charges (see the 2010/11 tax rates guide).
If you're struggling to repay, HM Revenue and Customs (HMRC) advises you to get in touch as it may ensure you're not hounded for payment.
The payment due is the second 'payment on account' for the 2009-10 tax year.
How the payment on account is calculated
Those who filed a 2008-09 tax return would have had to return it, and pay any outstanding tax for that year, by 31 January this year.
But most would also have been asked to make an additional two payments towards their 2009-10 bill, each known as a 'payment on account'.
The first installment was due on 31 January and the second is due by the end of this month.
Each payment is half of your previous tax year's bill. So if you paid £10,000 tax in 2008-09 you'd have owed £5,000 on 31 January and the same amount by 31 July.
Only those whose previous year's tax bill was more than £1,000 have to make payments on account, unless more than 80% of the previous year's liability was deducted by an employer.
If you don't / can't pay
If you miss the July deadline you'll pay interest on the outstanding balance at a 3% annual rate until paid, as long as the final bill is settled by 31 January next year.
Importantly, if you can't pay, the Revenue says you should call it. It says in some cases, it may waive or reduce interest charges as well as giving you more time to pay.
Tax experts, from past experience, doubt you'll get any charges back, though they say you may get an extension on the deadline.
In addition, you can choose to lower your payment on account if you earned less in 2009-10 than in 2008-09. However, if after filing your 2009-10 return, you still owe money, you'll face interest charges on the difference.
Further reading/Key links
Full tax breakdown: 2010/11 tax rates