Plans to part-privatise the Royal Mail have been condemned by a powerful cross-party Commons group that includes Kingswood Labour MP Roger Berry.

The Business and Enterprise Committee, which has published a report on the proposals, said it did not believe the Government had made its case to sell off around one-third of the postal group to a private firm.

Mr Berry, who raised the alarm about what he said were gaping holes in the scheme in the Post last week, said despite Ministers insisting handing over some control was the only option, they did not have any figures to back up their plans.

He said: "How much does Royal Mail need? All we know is that Royal Mail's chief executive, Adam Crozier, has said 'hundreds of millions'.

"And where are the companies falling over themselves to buy equity in Royal Mail?"

The Government has also failed to set out how much the sale would raise and what it would do with the money and committee members branded the plans "as clear as mud".

MPs also criticised the decision to sell during the middle of a recession.

Chairman Conservative Peter Luff said MPs agreed Royal Mail's multi billion-pound pensions deficit had to be tackled and the regulatory system reformed, but it "did not follow" that a part-sale was necessary.

He added that the assessment used to justify the sell-off was either badly drafted, or there was a "smoking gun" behind the proposals. He said: "We are being asked to give away a lot without any assurances."

The Bill is not expected to be discussed by the Commons until June.