The partial sell-off of Royal Mail is opposed by MPs of all parties, which could spell defeat for the Government, a survey for The Independent has found.
As Labour rebels warned that the proposals would spell "electoral suicide" for the party, the ComRes survey showed that their doubts are shared by some Tory and Liberal Democrat MPs.
That could make it harder for ministers to rely on the votes of Tory MPs to force its plans through Parliament even though the Opposition backs them.
Lord Mandelson, the Business Secretary, will publish a Bill tomorrow to allow the Government to sell about a third of its stake in the Royal Mail. But the move has been condemned by 125 Labour MPs.
The survey of 154 MPs from all parties found that only 28 per cent of Labour backbenchers back the Government's line that a part-privatisation deal is the only way to safeguard the long-term future of the Royal Mail, while 58 per cent oppose it – a margin of more than 2-1 against. There is more support among Tory MPs, 64 per cent of whom support the Government's approach. But 28 per cent oppose it, suggesting that some Tories may join with the Labour rebels to oppose the Bill. Among the Liberal Democrats, 53 per cent endorse the partial sell-off while 43 per cent reject the idea.
If the survey's findings are reflected when the Commons votes on the sell-off in the next few months, the Government would be defeated by 47 per cent to 39 per cent.
Labour MPs joined more than 500 postal workers as they staged a lobby of Parliament. Billy Hayes, general secretary of the Communication Workers Union, said: "Today is about people's democracy against the political elite."
Michael Connarty, a Labour MP, warned that the plans would "divide the Labour movement down the middle" and result in the party "committing suicide in front of the electorate".
Another Labour MP, John Grogan, claimed that at least three cabinet ministers were opposed to the sell-off. "I know because they have spoken to me and told me they are," he said.
But Lord Mandelson insisted: "The top and bottom line is that our policy will keep Royal Mail in the public sector and our legislation will make this clear. But Royal Mail will run out of money to sustain its current universal, six-day service unless its pension fund deficit is solved and its business transformed."
Adam Crozier, Royal Mail's chief executive, told the Commons Business Select Committee the group faced declining mail volumes and an increasing pensions deficit, adding: "We are going to have to do some pretty difficult things otherwise the company will not survive."
Kenneth Clarke, the shadow Business Secretary, said the Tories were ready to provide the Government with a majority. "We will support the part-privatisation plans, because what the taxpayer needs is a fully modernised and efficient postal service that can compete with the best in the world. It's the best way to secure a future for Royal Mail and make sure that we can send mail anywhere in the country for the same price," he said.