The majority of the nine million taxpayers who file a self-assessment return will have to make a payment by 31 July.
If you forget or can't make a payment by the deadline you face interest charges but you won't be hit with exorbitant penalties.
What's due is the second 'payment on account' for the 2011-12 tax year.
What is this charge?
Those who filed a 2010-11 tax return would have had to return it, and pay any outstanding tax for that year, by 31 January 2012.
But most would also have been asked to make an additional two payments towards their 2011-12 bill, each known as a 'payment on account'.
The first instalment 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 2010-11 you'd have owed another £5,000 on 31 January and the same amount by 31 July to cover the 2011-12 payment on account.
You will then need to file a self-assessment form for 2011-12 by 31 January next year, and pay any outstanding tax by that date.
If the payments on account were more than the tax owed you'll get a refund.
If you don't or can't pay
If you miss the July deadline you'll pay interest on the outstanding balance at a 3% annual rate until paid, but no more, as long as the final bill is settled by 31 January next year.
If you can't pay, let HM Revenue & Customs know before the end of the month to avoid it chasing you.
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.
In addition, you can choose to lower your payment on account if you earned less in 2011-12 than in 2010-11. However, if after filing your 2011-12 return, you still owe money, you'll face interest charges on the difference.