MPs 'set to get vote on EU referendum'
Eurosceptic Conservative MPs are trying to engineer a Commons vote next week on an EU referendum before 2015.
They have tabled an amendment to the motion welcoming the Queen's Speech, expressing regret that it did not include a referendum.
MP John Baron said he wanted ministers to remain "focused on this issue".
Mr Cameron has promised a referendum after the next election, if the Tories win. His spokesman said he was "relaxed" about the amendment.
There was no mention of Europe in this week's Queen's Speech, speech setting out government plans.
BBC political editor Nick Robinson understands the Speaker is to accept the amendment, so it could be debated and voted on next Tuesday or Wednesday.
Even if it was supported by a majority of MPs, it would not force the government to table a bill to bring in a referendum.
But Mr Baron said: "It would send a clear message that we are not going away and that there is a large body of opinion inside and outside this place that believes that legislation is right for a EU referendum."
Peter Bone, who tabled the amendment alongside Mr Baron, said he had already secured the backing of a number of Tory MPs. MP Nadine Dorries said she would sign it, on her first day back in the party having had the Conservative whip suspended for six months.
A group of Tory MPs has been calling on Mr Cameron to legislate in the current Parliament for a referendum on whether Britain should remain a member of the EU, potentially binding the next government to holding one.
Some have even called for a referendum before the next general election.
Mr Cameron has promised to hold a referendum in 2017, if the Conservatives win the general election and after he has secured the return of some powers from Brussels.
But the surge in support at last week's local elections for UKIP, which wants Britain to leave the EU, has added to pressure on Mr Cameron from within his own party.
At a conference in London on Thursday, Mr Cameron said it was "logical, sensible, practical" to first seek to renegotiate the terms of the UK's relationship with the EU before holding a referendum after the 2015 general election.
"I am faced as I do so, if you like, by two groups of pessimists," he told the conference.
"There are some pro-European pessimists who say, 'You have to, in Europe, simply sign-up to every single thing that anyone in the EU suggests. You sign every treaty, you sign everything - there is no alternative'.
"I think they are completely wrong.
"The second group of pessimists say there is no prospect of reforming the EU, you simply have to leave. I think they are wrong too.
"I think it is possible to change and reform this organisation and change and reform Britain's relationship with it."
But his policy has been questioned by former Conservative Chancellor Lord Lawson. And another former senior Conservative, Michael Portillo, described it as an "insincere ploy", arguing the EU no longer served the UK's interests.
And on Thursday, another former Chancellor, Lord Lamont, told BBC Radio 4's World At One programme: "I basically agree with Nigel Lawson, with one reservation.
"I think that the economic advantages of the EU are vastly over-stated. I think we could manage on our own, as Switzerland - much more integrated with the EU than we are - does.
"The only point on which I would disagree with him is that I think it is perfectly logical, and reasonable actually, to make one final attempt to renegotiate our relationship."
But he said that renegotiation should be far-reaching.
"Basically, I would want to be out of almost everything other than just having a commercial relationship with Europe," Lord Lamont said.
"Europe has changed and become more and more a political union, a political organisation.
"There's a mismatch with Britain. It's inconsistent with our parliamentary accountability and democracy. I don't think we are at all happy with it."
But the Conservative Mayor of London, Boris Johnson told reporters: "My view has always been narrowly in favour of staying in, particularly for protecting British interests in the single market to avoid us being discriminated against.
"I now think that there is a real opportunity to get a better deal from Europe and that is what we need to do. And that is why the idea of a renegotiation with Europe is a very good one."
But he said if negotiations did not go well Britain must also be prepared to "walk away".