Around six million people have paid the wrong amount of tax under the Pay As You Earn (PAYE) system, according to Her Majesty's Revenue and Customs (HMRC).
In the last two years approximately two billion pounds has been underpaid to HMRC under the PAYE system, while £1.8 billion has been overpaid.
Because of the errors around 1.4 million people will have to pay an average of £1,500 to make up for unpaid tax, while 4.2 million people are expected to get a rebate worth an average of £418.
Tens of thousands of people are to receive letters next week informing them of whether they have paid too much or too little. Those who have underpaid are to have their tax code changed next year in order for HMRC to receive the unpaid taxes.
Soon after its formation, the Coalition government began a review of the PAYE system.