The first letters from HM Revenue and Customs will arrive on doormats this morning, as nearly six million people discover that they have paid the wrong amount of tax.
For some, the cheque in the post will be a welcome surprise, but for those who have underpaid, the average bill will amount to £1400.
Paul Lewis, of Radio 4's Moneybox programme, advised listeners that they did not have to accept the bill if they had done everything right on their tax returns.
"If they ask you for money, I think you should challenge it," he said.
The erroneous payments were discovered when a new computer system checked past tax records.
But Conservative MP Michael Fallon said the computer, which has also made mistakes of its own, only masked an antiquated tax collection system.
"The Revenue should be an organization one can trust, it shouldn't be making mistakes like this".
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