The Social Security Select Committee defied ministers by issuing the results of an official pilot study into the pounds 2.5bn family credit system.
Alistair Darling, the Secretary of State for Social Security,had warned the committee not to publish the results as they were not statistically valid and were never intended to be made public.
However, the committee said it was acting to promote greater government openness and better-informed debate and attacked the Benefits Agency for failing to use the study to carry out a wider inquiry.
The nationwide study found that 123 out of 326 "outcomes" of case reviews involved possible or actual fraud.
"We remain concerned that these indicative findings did not lead on to a full review, which could have analysed the probable extend of each type of fraud," the committee's report into the affair said. "It is a matter for regret that the Benefits Agency did not proceed to a full review of family credit based on a sample large enough to give robust results."
The MPs said that the figures were important to establish a baseline of fraud levels for Parliament to judge the Government's plans to introduceworking families tax credit. Iain Duncan Smith, the Conservative's social security spokesman, said: "Labour promised to be tough on benefit fraud, but this report shows their utter complacency.
"The fact that the Government delayed publication of this report proves their failure to deal with fraud. The situation is bad enough, but things will get even worse with the introduction of the working families' tax credits, which will encourage greater fraud as it places more emphasis on speed of payment at the expense of careful checking."
The Liberal Democrat's spokesman on tax credits, Steven Webb, said the report showed the Government was "ducking the issue on fraud".
"If the Government really did want to stop fraud, it would continue this review and use the results to ensure that the tax credit was watertight and built to prevent rather than encourage fraud," he saidReuse content