Labour's Kingswood MP has launched a fierce attack on his government's plans to part privatise Royal Mail.

Roger Berry said proposals drawn up by Business Secretary Peter Mandelson go directly against the party's manifesto pledge at the last election. While he concedes the service faces major challenges, the proposals would fail to make it through entrepreneurial television show Dragons' Den, he insisted.

"The fact of the matter is that there is no business plan on which we can judge the Government's proposal. Or, if there is, it is not in the public domain. Parliament is being asked to agree a pig in a poke," he said.

He added: "The Labour Party manifesto of 2005 said, with reference to the Royal Mail, 'We . . . have no plans to privatise it. Our ambition is to see a publicly owned Royal Mail fully restored to good health' . . .

"More recently, at Labour's annual conference in September last year – just five months before the Secretary of State for Business and Enterprise, Lord Mandelson, announced his part-privatisation plans – delegates endorsed the statement: 'We have set out a vision of a wholly publicly owned, integrated Royal Mail Group'. This set of proposals would certainly not make it through Dragons' Den."

Mr Mandelson's plans to hand control of the service follow the recommendations in the Hooper Report, an independent review of the UK postal services sector commissioned by the Government.

It recommended the government taking over responsibility for the £9 million Royal Mail Pensions Fund's deficit and inviting other postal or network companies "to come forward with proposals for developing a strategic partnership with the Royal Mail, including taking a minority stake in the company's letter and parcels business".

On Tuesday the Business and Enterprise Committee, of which Mr Berry is a member, will publish a report on postal services, which is likely to be critical of the plans.

Mr Berry, a committee member, added: "What I find entirely unconvincing are the arguments that a "partnership" must take the form of part privatisation of the Royal Mail and that the three elements of the package - reforming regulation, the Government taking responsibility for the pension deficit and part privatisation - must be taken together as the only alternative to the status quo.

"The proposed part-privatisation of Royal Mail does not seem to many to make the best of sense at any time. However, when the private sector is seeking unprecedented investment from government because it can't generate the finance itself, it is an odd time to argue that selling minority equity in Royal Mail to a private shareholder is the key to success."