Millions of pounds of benefits have been given to prisoners even though they are not entitled to assistance, government figures reveal.
Prisoners are not entitled to a range of benefits
The Department of Work and Pensions said prisoners received £13 million in income support and Jobseeker's Allowance over the past three years.
Offenders are meant to have their benefits automatically stopped when they are sentenced.
The government said it would now tighten up its procedures for benefits.
The figures, released after Conservative Work and Pensions spokesman Philip Hammond tabled a Parliamentary question, show the total payout to prisoners in 2004-5 was £7 million - more than three times the amount paid out in 2002-3.
The department said it was a small part of overall benefit fraud, which involved an estimated £290 million in 2004-5 out of a spend of £110bn.
Prisoners are not entitled to claim Jobseeker's Allowance - worth up to £57.45 a week - because they are not available to work while locked up.
They are also ineligible for income support and pension credits, and generally cannot receive incapacity and disability benefits, the state pension, carer's allowance, industrial injuries benefit or maternity allowance.
Housing benefits are only given to those on remand awaiting trial.
Parliamentary under-secretary of state for work and pensions James Plaskitt said in his written reply that the government did not know how much had been wrongly issued in incapacity benefit, council tax benefit or housing benefit.
He also said there were "no estimates available" for how many prisoners had received money.
Mr Hammond said a full investigation was needed.
"It's really a slap in the face to law-abiding taxpayers and to the victims of crime, that prisoners who are locked away for the crimes they've committed are able to perpetrate a fraud on the benefits system."
A department spokeswoman said the figures were estimates, based on small samples which had a "wide margin for error".
"We are determined to crack down on anyone who defrauds the benefit system. The idea that we are in any way complacent about this is just wrong.
"We are the first people to start counting it."