Labour backbenchers are threatening to defeat the government over plans to part-privatise Royal Mail.

An early day motion tabled by Geraldine Smith has attracted the signatures of 66 backbench Labour MPs. If all vote against the government and the proposals are not supported by opposition parties Gordon Brown could suffer his first major defeat in parliament.

Frank Dobson, Michael Meacher and Malcolm Wicks are among the ex-ministers who have signed the motion but business secretary Peter Mandelson remains determined to push through his proposals.

He told BBC1′s Breakfast programme this morning there was “no question” of privatising Royal Mail.

“What we want to do is to introduce a minority stakeholder – I mean an experienced postal operator from somewhere else in Europe – to team up with the Royal Mail to bring much-needed investment for modernisation and increased efficiency, but also management expertise to enable the Royal Mail to become strong and viable and a real postal player in the future,” he said.

Last month saw Lord Mandelson indicate Dutch postal services firm TNT would be favoured by the government if it was interested in making a bid.

His decision to back the findings of an independent review into Royal Mail’s future led to the resignation of Jim McGovern, a junior member of the government.

The motion tabled by Ms Smith, the Labour MP for Morecambe and Lunesdale, is deliberately targeted at Labour MPs as it notes the “vision of a wholly publicly-owned, integrated Royal Mail Group” at the party’s 2008 autumn conference.

And it makes clear its opposition to Lord Mandelson’s proposal when it states it “rejects the recommendation of the Hooper report to sell a minority stake in Royal Mail which would risk fracturing one of Britain’s greatest public services”.