People hit with an unexpected tax demand may be able to refuse to pay up as HM Revenue & Customs could have exceeded its own time limits in which to ask for the money, experts have said.
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 people had underpaid.
It emerged over the weekend that nearly six million people have paid the wrong amount of tax through the Pay As You Earn (PAYE) system.
Around 4.3 million of these have paid too much and are due a refund, but 1.4 million have underpaid and will have to hand over an average of £1,428 each.
But if people provided HMRC with all the information they needed to get their tax code right, it should have used this information within 12 months of the end of the tax year in which it was received to claw back the 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.
Angela Beech, partner at chartered accountants Blick Rothenberg, said: "Those that receive these demands need to think before they automatically pay up.
"If you had given HMRC information that would have enabled them to adjust your tax code to make sure that you did pay the right amount of tax, then, if the time limit has passed for them to use that information, they cannot pursue you for the unpaid tax."
HMRC has begun sending 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.