The following correction was printed in the Guardian's Corrections and clarifications column, Wednesday August 13 2008

To clarify the article below, the one in five proportion was of cases in which paternity was resolved through DNA testing in 2007-08; fathers were wrongly identified in 0.2% of overall cases processed by the CSA during that period, or one in 500. Our headline should have said 'One in 500 fathers wrongly identified by mothers in Child Support Agency claims'.

· This article was further amended on Wednesday September 10 2008. We have now amended the headline in line with the above clarification.

Nearly one in five paternity claims handled by the Child Support Agency end up showing the mother has deliberately or inadvertently misidentified the father, figures show.

Since DNA paternity testing figures began to be collected in 1998-99, 4,854 paternity claims have turned out to be false after DNA testing.

Under child support legislation it is a criminal offence to make a false statement or representation, and to provide false documents or information.

However, according to the CSA, there has not been a single prosecution of a woman for making a false claim. The figures showing the number of false paternity claims have been compiled using freedom of information legislation.

The latest figures for 2007-08 show that out of 3,474 tests ordered, 661 or 19% named the wrong man. The figure is a record for the CSA since central figures started to be collated nationally. The negative results for tests taken in 2004-05 were 10.6%, in 2005-06 were 16.4%, and 13.6% in 2006-07.

Government-approved paternity tests, based on samples in multiple parts of the body, are deemed to be 99.99% accurate.

The CSA does not have figures for whether any women have named the wrong father on more than one occasion. They also appear to have no information as to why women named the wrong father.

CSA rules state that if the DNA test establishes that the named father is the actual father, then he must pay for the cost of the test. If the DNA establishes he is not the father then the taxpayer pays, so there is no consquence for the mother in making a false claim.

The government has spent £9.37m on paternity tests since 1998. This includes refunds for DNA tests privately funded.

Under CSA rules, men must pay maintenance the moment they are named as the father of the child. They can challenge the ruling by asking for a DNA test but have to pay for it themselves.

Chris Grayling, Conservative spokesman for work and pensions, said yesterday: "This is an extremely worrying trend and one where proper action should be taken. If some CSA claimants are getting away with making false applications, it will not only slow things down for other families, but it also sends the wrong message about the things we're willing to accept."

The revelations came as CSA figures this week showed the much-criticised and reformed agency has been unable to collect £3.8bn in outstanding money owed by non-resident parents to parents who care for the child. The figure represents an increase of £120m on last year. Around £2.2bn of the debt is "probably uncollectable", and the overall figure is rising by £10m a month, the statistics showed.

But the CSA said the latest performance figures showed 768,000 children were benefiting from maintenance payments - an extra 65,000 over the previous year and an increase of 207,000 since March 2005, before the agency's improvement plan was launched.

Of the £1.05bn benefiting children - an increase of £252m since March 2005 - a total of £137.6m collected was arrears. Both figures represent the highest level of money ever collected or arranged in a 12-month period.

At the same time, research by Grayling shows that more than 6,200 non-resident parents registered with the CSA are living abroad, but only 25% of them are paying any child maintenance. Of the cases abroad 5,000 owe a total debt of more than £26m, an average of more than £5,000 each.

There are 121,000 uncleared applications to the CSA. The agency now costs more than £500m a year to run, more than double the cost in 1997. It cost £563m in 2007/08 compared with £520m last year and £226m in 1997-98.

DNA testing

Paternity tests are relatively simple. In the past, blood samples were tested, but most now involve using a swab to take samples of cells from inside the cheek. The mother, child and assumed father must all take the test and the same type of test, whether blood samples or cheek cells. After examining the individual genetic markers in DNA, scientists can give an answer that is more than 99% accurate. Paternity tests are not available on the NHS. The tests, for mother, child and assumed father, usually costs £257.58, according to the NHS Direct website.

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