Tory leader David Cameron has pledged to force a Commons vote on whether a referendum should be held on the new European Union treaty.
Mr Cameron called on Labour MPs to back his motion
Mr Cameron said he would "push the government all the way" by holding an Opposition Day debate on the issue shortly after MPs return in October.
TUC delegates are due to vote on the issue on Wednesday - motions from the RMT and GMB back a referendum.
The government insists that a referendum is not necessary.
Gordon Brown says it is not the same as the constitution - on which the Labour government promised a referendum, until it was rejected by Dutch and French voters in 2005.
But Mr Cameron, in an interview for Telegraph TV, said Labour's manifesto pledge had been to hold a referendum and they should not be allowed to "break that promise".
He said the treaty was about "passing powers from Westminster to Brussels" and said: "We will push the government all the way on this issue.
"When Parliament returns we will hold an Opposition day debate where we will put down a very simple and clear motion that we support a referendum on the European constitution."
He added: "Labour MPs should stick to what they promised. They should vote for our motion when we bring it forward in the House of Commons."
A cross-party campaign for a referendum, which includes some Labour former ministers, was launched last week.
On Tuesday the TUC's ruling body failed to agree a statement on the EU treaty which could have headed off a vote on whether to hold a referendum.
Union delegates at the TUC conference now look set to vote on Wednesday, on motions from the RMT and GMB urging a referendum.
Both say the treaty is almost the same as the discarded EU constitution, but they differ on what action to take.
The GMB is calling for a vote in order to push the government to end its opt-out from European social legislation.
But the RMT's motion goes further in demanding that the TUC should actively campaign for a "no" vote in the event of a referendum.
If either motion is passed by the TUC, it will add to the pressure on the prime minister to hold a public vote.
The UK Independence Party and a number of Labour and Lib Dem MPs are also calling for a referendum.
Last month, Mr Brown said: "The proper way to discuss this is through detailed discussion in the House of Commons and the House of Lords."
He added that he was confident Parliament would pass the treaty.
The government argues that it differs substantially from the EU constitution.
Europe Minister Jim Murphy told the Guardian that the EU treaty was a "very different deal" and was "more different" for the UK than for any other member state.
"The reform treaty confirms that we will continue to control our foreign and defence policies. The constitution didn't spell this out," he said.
But opponents say the documents are largely the same in substance and that the promise of a referendum, made ahead of the last general election, should still apply.