Millions of Britons will have to return money to the government after HMRC made a big Pay As You Earn (PAYE) blunder last year.
Over 3.5 million people have paid too little tax and will have to pay back over the next few years.
Two million taxpayers paid too much of tax and will be able to claim it by April next year.
The estimated average mistake for the taxpayer is £300.
The errors are particularly embarrassing for the Treasury as it introduced a £270m scheme to make the tax system more effective.
A spokesman for HMRC defended the current system: “Most people pay the right tax throughout the year, but there will always be a small percentage of the 41 million people in PAYE who have underpayments or overpayments at year end.”
He added: “The effect of Real Time Information is not reflected yet as it has not bedded in but, over time, RTI will help to reduce the number of cases that have to be reconciled.”