About 1.6 million people will start receiving demands within the next two weeks for an average £537 shortfall in the tax they paid last year, HM Revenue and Customs warned yesterday.
Meanwhile a further 3.5 million will be sent a rebate for the 2011-12 tax year, averaging £379.
If the figures are correct for the tax year which ended on 5 April, then Britons overpaid more than £1.3bn in tax for the year. Meanwhile, HMRC's miscalculations means it will be forced to claw back more than £849m from unsuspecting taxpayers.
The only consolation for those facing new tax demands is that they may be able to spread the cost of the fresh tax demand through their normal PAYE tax payments.
Computer problems have beset the government agency since a new PAYE system was introduced in 2010.
Stephen Banyard, the acting director general for personal tax at HMRC, said: "We have started this process [of adjustment] more quickly than in previous years, giving certainty to those with something to pay."
The demand letters will start going out on 17 May, with the last being sent in October.