Thousands of Oxfordshire families are being forced to pay back tax credits of around £1,000 each after a Government blunder.

The scale of the fiasco has been laid bare by a Government report which reveals 14,600 claimants in the county were overpaid last year and must collectively reimburse the Treasury for £15.7m.

The average Oxfordshire family will have to pay £1,075 as a result of the chaos which stems from the introduction of tax credits for working families with children in 2003.

The newly-published figures, compiled by HM Revenue and Customs analysts, reveal that more than a third of Oxfordshire's 49,000 claimants were overpaid because of a flawed calculation system.

Another 6,200 claimants were left out of pocket to the tune of £4.1m because they were not paid enough, the report found.

Mike O'Donoghue, a welfare benefits officer for the Oxford Citizens Advice Bureau, said it dealt with many people who had been overpaid and were struggling to pay back money they owed.

He said: "People on income support are dependent on child tax credits. It can be quite serious for single parents expected to cope on a lower income."

The Inland Revenue calculates how much tax credits are given according to the previous financial year, causing overpayments to be made if people's circumstances have changed, he said.

If income changes within the financial year, the Inland Revenue tries to collect overpayments back that year. Mr O'Donoghue added: "Common reasons for a change of income are splitting up with a partner, changing jobs, going on long-term sick or maternity leave or becoming unemployed, which can cause an overpayment or an underpayment,"

"A lot of problems are caused by official error. Sometimes the Inland Revenue has the correct figures but inputs them wrongly.

"People have no right to appeal but if they have been overpaid because of an official error, and had no way of knowing they were getting too much, they can fill in a form TC846." Nationally, the Government is trying to reclaim £2bn in overpayments up to April last year.

Last year, a computer error meant 455,000 people were overpaid and many had spent the money before being told they had to give it back.

The Government says overpayments have been made because recipients have not told them about changes in their circumstances.

But a spokesman for HM Revenue and Customs admitted computer errors were also to blame.

He said: "When the overpayment is the result of our mistake, and the claimant could reasonably have thought the award was correct, then we can write it off."

Treasury Minister Dawn Primarolo has unveiled a six-point plan aimed at tackling the crisis.

She said cash recovery would be suspended in genuine cases of hardship during disputes between HM Revenue and families it had allegedly overpaid.

Concerned families can contact the Tax Credit Helpline on 0845 300 3900 or the Oxford Citizen's Advice Bureau on 0870 2200608.