Millions of people face having to hand money back to HMRC after errors meant they ended up paying the wrong amount of tax on their earnings.
It is estimated that 5.5 million people have paid the wrong amount of tax, with 3.5 million thought to have paid too little and the remaining 2 million having overpaid, meaning they can claim a refund.
The mistakes can occur due to a change in personal circumstances, such as if a person moves jobs or starts receiving benefits.
According to tax officials cited by the Daily Telegraph, the average size of the error is about £300.
This is despite the introduction of a new £270 million 'Real Time Payments' scheme designed to make the tax system more accurate by letting people update their information on a weekly or monthly basis.