The Child Support Agency has stepped up enforcement efforts and is increasingly using private companies to collect unpaid money, the BBC has learned.
The CSA has been criticised by parents
The number of parents going to prison for non-payment of child maintenance also rose to 22, from six in 2005.
The figures come as the agency, which has been dogged by problems and is owed £3.5bn, is to be scrapped.
Campaigners fear that more than £1bn owed to parents will be written off when the agency is replaced.
The details of the agency's replacement, which will have more enforcement powers, are set to be announced next week.
The CSA, which was set up 13 years ago, said its use of debt collectors had enabled it to recover about £320,000 which it would not otherwise have recovered.
It has transferred about 10,500 cases to the private sector.
BBC political correspondent Gary O'Donoghue said these cases were worth an estimated £45m of the total debt.
In July, the CSA had a backlog of 300,000 cases and was the subject of many complaints from parents.
The Child Support Bill, unveiled in the Queen's Speech, aims to replace it with a smaller body and a simpler way of collecting child maintenance.
Work and Pensions Secretary John Hutton has said the government will seek more powers to deal with parents who repeatedly fail to pay maintenance - such as suspending passports and imposing curfews.
But campaigners are worried that tens of thousands of parents will see money owed to them in child support written off.
James Price, of the Family Lawyers Group, said: "We are really worried the government is going to write off £1bn of debt and that will affect tens of thousands of families and encourage non-payment in the future.
"There have been so many mistakes in the past which the government hasn't learned from."
Jacqueline Harthill, who says she is owed £10,000 in child support payments for her daughter, said she was "outraged" at the prospect of having the money written off.
"I've had to re-mortgage three times just to try to stay afloat and they are telling me they might write off this £10,000 which might make a difference," she said.
"This is just so unfair. It's not their money, it's not even my money. It's money that is actually owed to my daughter and I'm just outraged."
James Pirrie, of the Credit Services Association, the UK's national association for debt recovery agencies, said: "There have been major problems sorting the type of debt, how much and whether or not these amounts are actually true.
"Of course there's tracing issues as well where people have left their addresses and the information that we have is not correct information in the first instance."