The Tories would hold a referendum on the EU treaty, if they won power before it was ratified by all EU states.
Mr Cameron wants a referendum on the new Lisbon Treaty
Party leader David Cameron said even if Parliament ratified the treaty, a Tory government would hold a referendum.
He admitted it would be difficult if all 27 states ratified it without a referendum, but said he would not be happy to let "matters rest there".
The government says a referendum is not needed as it has secured opt-outs and most changes are minor or procedural.
Gordon Brown signed the treaty, as did other EU heads of state, in December. But it has to be ratified by all EU parliaments, before coming into force - something which is expected by 2009.
But Mr Cameron said while it was still being discussed, he wanted to keep the pressure on for a referendum.
Asked whether he could offer a "cast iron" guarantee that he would offer a referendum when in government, he said: "As soon as we have an election, the sooner we can have a referendum".
He said a referendum could take place "absolutely no problem at all" while it was still being discussed in Europe. But he admitted it was more difficult if there was no general election until 2010 and if all member states had ratified it without holding a referendum.
"We will not be content to rest at that point because we think too much power will have been passed from Westminster to Brussels," he said.
"But I don't want to explain exactly what we would do in those circumstances because we've got to wait and see whether they take place."
He said he did not want to leave the EU as it was in British interests to be part of it but accused Labour of "lying down and taking whatever is suggested in Brussels".
He added: "I think Margaret Thatcher showed in recovering the British rebate that if you have a very clear, very straightforward approach in Europe and say look, we want to be in the European Union ... but we're not happy with the status quo and there are some things we want to change, if you're single minded about that then there's no reason you cannot achieve your objectives."
But Europe Minister Jim Murphy said Mr Cameron's policy was in "complete confusion" - as he had not explained what he would do if the treaty was ratified by every other state, without a referendum.
He added: "Attempting to reopen negotiations on a treaty ratified by every member state would put the Tories into the extraordinary position of repudiating an international treaty, reopening and renegotiating Britain's terms of EU membership."
The Tories say Labour promised a referendum on the Constitution and should honour that promise as the treaty is essentially the same thing.
But the government says it is substantially different and previous, more ambitious treaties like Maastricht, did not go through referendums.
The treaty has to be ratified by Parliament and the government faces opposition from the Tories, as well as some Labour and Lib Dem MPs.
The treaty, which is to be debated line-by-line during February and early March, will also face a battle to get through the House of Lords.