The Conservatives are "unlikely" to defeat the government over a referendum on the EU reform treaty, shadow foreign secretary William Hague told the BBC.
Mr Hague accused the government of breaking its "solemn" promise to hold a public vote - and pledged a "real Parliamentary battle" to win one.
But he acknowledged it would be "difficult" to defeat Labour without the backing of Lib Dem MPs.
Both Lib Dem leadership contenders have said they are against a referendum.
The Conservatives claim as many as 120 Labour MPs are in favour of one - easily enough to overturn Labour's 69 majority.
But the Tories concede the number of Labour rebels could fall significantly with pressure from government whips.
Mr Hague told BBC One's The Andrew Marr Show: "If the Liberal Democrat MPs also break their solemn election commitment, as well as Labour MPs, then of course that makes it difficult for us to win a vote in the House of Commons.
"Although those in marginal seats will have to remember that their breach of that promise is something we will remind their constituents about in some detail at the next general election.
"There will also be a debate in the House of Lords. These are very difficult to forecast and there is public pressure on the government."
He added: "It is very much worth fighting for a referendum now and that is a debate we will intensify over the coming weeks.
"There are going to be real parliamentary battles about this treaty, which is the European constitution in disguise... Nearly everything that was in the constitution is back in it."
Alliance ruled out?
If the Conservatives lose the fight for a plebiscite on the treaty they would then have to decide whether to fight the next general election on a promise of a referendum to rescind it, said Mr Hague.
Speaking earlier on GMTV's Sunday programme, shadow Europe minister Mark Francois said Lib Dem MPs were "key" to Tory hopes of defeating the government in the Commons.
But any prospect of a Lib Dem/Tory alliance appears to have been ruled out by the two contenders for the Lib Dem leadership, Nick Clegg and Chris Huhne.
Asked by the BBC's Andrew Marr if they would back a referendum, both men said: "No".
Europe Minister Jim Murphy told the Sunday Telegraph he was "confident" the government will achieve its goal of ratifying the treaty without holding a public vote, predicting pro-European Tories would back the government.
The reform treaty was agreed in the early hours of Friday at an EU summit in Lisbon, but all 27 EU member states must ratify it by 2009 for it to come into effect.
So far, only the Irish Republic has said it will hold a referendum, with other countries, including Britain, planning to ratify the treaty through their national parliaments.
Prime Minister Gordon Brown has promised it will be given the fullest scrutiny by MPs before being put to a vote - a process that could take up to three months.
Europe Minister Jim Murphy hit back at suggestions the Conservatives could rescind the treaty after a public vote.
"If William Hague got his way and this treaty were renegotiated, Britain would be left isolated with no allies in the EU. The EU would waste 10 years on instability and uncertainty.
"Unfortunately, Mr Hague is talking from political calculation not patriotic conviction. This is not a serious party ready for government."
In a statement responding to Mr Hague's comments, Mr Murphy added: "The Reform Treaty gets the EU's machinery under control and fit for 27 members.
"It confirms the UK vision of an EU of sovereign member states working together as the way forward. "So the new President of the Council is appointed by and is accountable to the member states.
"The same goes for the High Representative for foreign policy.
"These reforms mean more coherence, not more centralisation."