Six million people in UK have overpaid or underpaid tax
Nearly six million people in the UK have paid the wrong amount of tax.
About £2bn was underpaid via the Pay as You Earn (PAYE) system in the past two years, with about 1.4 million people owing an average of £1,500 each.
But £1.8bn has also been overpaid and some 4.3 million people will get a rebate because they have paid too much.
Treasury minister David Gauke said that in the current financial climate, the government was not in a position to "just wave goodbye" to the money owed.
He said the government had inherited the problem and the PAYE system - which was created in the 1940s - was struggling to cope with modern working patterns.