Thousands of taxpayers across Northern Ireland are bracing themselves this morning for a letter from HM Revenue & Customs demanding additional payment.

Taxpayers in the province are among the 1.4m people in the UK who may have paid too little income tax in recent years.

It was revealed on Tuesday that around 5.7m people have paid the wrong amount of tax.

Treasury minister David Gauke said those affected by a HM Revenue & Customs blunder should not “build up their hopes” that appeals will succeed.

The intervention came after HMRC started sending out nearly six million letters to individuals who paid too much or too little income tax.

Around 4.3m people are due a rebate, but some 1.4m will have to hand over an average of £1,428 each.

Experts said people hit with an unexpected tax demand may be able to refuse to pay up as HMRC could have exceeded its own time limits. Under tax rules, HMRC must issue demands for underpaid tax within 12 months of the end of the tax year in which it became aware that it was underpaid.

But if people provided all the information needed to get the right tax code, HMRC should have used those details within 12 months of the end of the tax year in which it was received to claw back extra money. If HMRC failed to do this, taxpayers can ask for an Extra Statutory Concession, also known as an ESC A19.

The latest round of errors date back to April 2008, meaning anyone who alerted HMRC to changes in their circumstances that affected their tax code before the start of the new tax year in April 2009 may be able to cite this clause.

But Mr Gauke insisted experience had shown that “very few” appeals against payment demands succeeded.

HMRC has sent out the first 45,000 letters to people who are affected, around 30,000 of whom are due a rebate, while 15,000 have underpaid tax. The remainder of people affected will be contacted before Christmas.

If you have received a letter from HMRC the Belfast Telegraph would like to hear your story. Call the newsdesk on 02890 264420.