Labour's Euro-rebellion against plans to impose a new constitution by the backdoor

Already 16 have signed Commons motions calling for voters to have a say.

Party sources said at least 25 more were preparing to back the demands.

Union leaders, meanwhile, made clear the referendum campaign would be a key issue at their annual conference next month.

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The draft agenda for the TUC meeting includes a prominent motion, tabled by the Rail, Maritime and Transport Workers Union, insisting the new treaty is 'substantially the same' as the previous EU constitution on which Labour promised a referendum.

It claims the blueprint will transform the EU into a 'state with a single legal personality', form a centralised EU government, reduce the power of member states to block EU legislation, 'militarise' the EU and give the EU the power to change itself without any further treaties or referendums.

Of 353 Labour MPs in Parliament, 170 are sponsored by the big three unions in favour of a referendum - the T&G, the GMB and Unison.

More than half of the Cabinet - including Gordon Brown, Alistair Darling, Jack Straw, Des Browne, John Hutton, Harriet Harman and Peter Hain - also have backing from these unions.

Senior Labour backbencher David Taylor said he would be joining the referendum campaign and urged the Prime Minister to think again.

"I admire Gordon Brown," said the Leicestershire North West MP.

"I have been encouraged by what he has done so far but on Europe he now needs to recognise the wind of change blowing through our party and the trade union movement."

Mr Taylor's declaration came as Europe Minister Jim Murphy insisted Parliament - not voters - would decide whether to sign up to the EU treaty.

Claiming the deal would be 'good for Britain', Mr Murphy said it would end 'navelgazing' over the EU's structures and decision-making processes and allow it to forge ahead with the 'big issues'.

In an article for the Epolitix website, he insisted: "Hyperbole and misunderstanding aside, the reform treaty will not transfer power away from the UK on issues of fundamental importance to our sovereignty.

"We will maintain national control over key areas including justice and home affairs, social security, tax, foreign policy, and defence."

Mr Murphy firmly rejected calls for a referendum, despite a Daily Mail poll this week showing more than four in five voters want one.

He said: "Our democratic system means that Parliament must ratify treaties.

"Parliament will therefore make the final decision on whether the reform treaty is in Britain's interests, just as it did with previous amending treaties under Tony Blair, John Major and Baroness Thatcher."

Tory Europe spokesman Mark Francois said: "Jim Murphy needs to remember that 80 per cent of the British people now want a referendum on the revived EU constitution.

"This includes his own trade union, the GMB, who helped him campaign at the General Election. He should honour the manifesto on which he and Gordon Brown were elected to Parliament."

Nigel Farage, leader of the anti-EU UK Independence Party, said: "Every other political leader in the EU has said that this is the constitution in all but name.

"He (Gordon Brown) is doing a rather splendid impression of an ostrich, surrounding himself with yes men who are trying to drown out the overwhelming calls for the British people to have their say."