One Labour MP said he was "astonished" at the move and warned it would unleash "mass opposition" in Parliament and among the public.
Gordon Brown will now have to tackle the biggest back-bench revolt since he became Prime Minister as well as opposition from postal workers and the prospect of a major union disaffiliating from the Labour party in protest.
Opponents of the Government's controversial plans had expected the Bill to be brought forward nearer to Easter, but it will be published on Thursday.
Hundreds of postal workers will join a massive protest tomorrow against the Government's plans to part-privatise the Royal Mail in a crucial week for all sides in the controversy.
Members of the Communication Workers Union from across the UK will travel to London for a rally before lobbying MPs as part of a campaign against selling off part of the postal group.
A Commons motion opposing the sale has been signed by 139 Labour MPs.
Billy Hayes, general secretary of the Communication Workers Union, said: "There is no support in Parliament or from the public for privatisation of Royal Mail. This is the wrong move by the Government."
The CWU, which has a policy to ballot members on disaffiliation from the Labour party should privatisation of Royal Mail take place, has warned that the Government's plans were a "defining moment" in its relationship with the unions.
The CWU gives around £1 million a year to the party as well as practical help such as supporting Labour during elections.
A poll of 1,000 members of the public for the union last month showed that three out of four disagreed with privatisation, rising to nine out of 10 when foreign ownership was mentioned.
Dutch postal group TNT has said it was keen to buy a stake in the Royal Mail, with other foreign firms from Denmark and Sweden reportedly expressing an interest.
Royal Mail chairman Allan Leighton and chief executive Adam Crozier are due to be questioned tomorrow about the future of the organisation at a hearing of the cross party Business Select Committee.
The controversy will also be raised at a crucial meeting this weekend of Labour's national policy forum.
Mr Hayes said privatisation of Royal Mail was a "totemic issue" for postal unions, adding: "The current government plans are a defining moment in the relationship between the unions and the government. The relationship will be severely strained towards breaking point.
"Coupled with planned job losses and office closures in Royal Mail, the government's actions are placing a huge strain on the unions' relationship with the Labour Party.
"The UK public is overwhelmingly opposed to government plans to privatise part of Royal Mail. This is a deeply unpopular move which would damage a trusted public service.
"We urge the government to take responsible action and respond to the justified concerns of UK citizens who do not want to see this valuable public asset carved up and sold off."
Deputy general secretary Dave Ward said: "Royal Mail is a profitable public company with a dedicated workforce and unmatched public trust. Why would you gamble this away on an unpopular and unproven part sell-off?
"A huge majority of the public is against this idea. It's time MPs listened to their constituents and scrapped this disastrous plan."
The Early Day Motion submitted by Morecambe and Lunesdale MP Geraldine Smith (Labour) "rejects" the recommendation of the Hooper Report to sell a minority stake in Royal Mail, warning it risked fracturing one of Britain's "greatest public services."
In December, Business Secretary Lord Mandelson endorsed the recommendations of the Hooper report which called for a minority stake in the business to be sold off.
He said Royal Mail could not survive in its current form, due to severe financial constraints and falling demand for sending letters, and needed fresh investment in technology to prosper.
The government denies the plan amounts to a sell-off of the business, saying it will be a "strategic partnership" which maintains Labour's commitment at the last election to keep Royal Mail in public ownership.
Labour MP John McDonnell (Hayes and Harlington) said: "This flies in the face of the manifesto commitment. Privatisation will result in significant job losses and attacks on the working conditions of postal workers and we will fight these proposals all the way.
"Labour Party members and trade unionists are angry at what will be interpreted by postal workers as a complete betrayal. Gordon Brown and Peter Mandelson should not underestimate the opposition that this will unleash among MPs, trade unions, postal workers and communities. This is a privatisation beyond what even Thatcher proposed.
"Labour MPs will be astonished that the Government has chosen to rush through this legislation without consultation and in breach of the manifesto on which they were elected."
Geraldine Smith said tonight that publishing the Bill was an "absolute disgrace" and smacked of Lord Mandelson trying to "pick a fight."
She said 140 Labour MPs had now signed her Commons motion opposing the Government's plans and warned ministers they will have to rely on Conservative support for the Bill to be approved.
"This is happening completely against the views of the Labour party. A private firm will put profit before service so there will be higher prices and the public will suffer.
"It is deliberate antagonism. Private firms are only interested in the Royal Mail because they think they can make a profit out of it, but privatisation has gone out of fashion so why is Peter Mandelson doing this?"
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