Gordon Brown is facing the biggest backbench revolt of his leadership over the part-privatisation of Royal Mail.

Former ministers Malcolm Wicks, Frank Dobson and Michael Meacher are among 66 Labour MPs who have signed an early day motion warning that selling a minority stake in the postal group would risk fracturing "one of Britain's greatest public services."

The motion is tabled by Geraldine Smith (Morecambe and Lunesdale), who said: "Royal Mail is one of the country's oldest and most respected public bodies. In recent years it has been exposed to ever-increasing, unfair competition by the regulator. Part-privatisation is not the answer."

Billy Hayes, general secretary of the Communication Workers Union, said: "The Royal Mail is a successful and vital public service which does not need private capital to ensure modernisation.

"We strongly welcome this Early Day Motion which reflects the opinions of millions of ordinary people. The public is fed up with privatisation and has suffered enough from flogging off public assets.

"The Labour Party has a clear commitment to a wholly publicly owned Royal Mail. This EDM is further proof of the commitment of Labour MPs to a modern public postal service."

The biggest revolt the Prime Minister faced before now involved 43 Labour rebels who wanted to make it easier for unions to comply with the law on industrial action.

Jim McGovern, a ministerial aide in Lord Mandelson's Business Department, resigned last month in protest at the prospect of a foreign firm taking a stake in the Royal Mail.

The Government was criticised by unions and Labour backbenchers after accepting a recommendation from an independent adviser that Royal Mail should "forge a strategic minority partnership" with a private company.

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