Media playback is unsupported on your device Media caption Ed Miliband challenges David Cameron over Europe policy

David Cameron has said he would fight for the UK national interest in any EU Treaty talks after being pressed on the issue by his own MPs at question time.

The prime minister said: "The more eurozone countries ask for, the more we will ask for in return", including safeguards about the financial sector.

But Labour leader Ed Miliband said the UK had been "left on the sidelines".

After the exchanges, London Mayor Boris Johnson called for a referendum if the UK was asked to approve a new treaty.

Conservative Mr Johnson said the UK should oppose any change which created a "very dominant economic government" across Europe.

"If Britain was asked to sign up to such a thing within the 27 (all the members of the EU), it would be right to veto it and if we felt unable to veto it, I certainly think that it should be put to a referendum," he told BBC Radio 4's World at One.

However, he said the government could not "reasonably" have a referendum if the new arrangements were confined to the 17 eurozone countries only.

Analysis It was once said that the five most frightening words a politician could hear were "Michael Crick is in reception". There's a new festive version of that for David Cameron: "Boris Johnson's on the radio". The Mayor of London popped up to make a characteristically eloquent contribution on saving the eurozone. Its survival is crucial for the UK economy and particularly the City of London investors, bankers, accountants and lawyers he represents. But he is worried about a democratic deficit. He said the UK should either veto major treaty changes that would create a fiscal union within the eurozone or hold a referendum. The former is very unlikely, given that the global economy is teetering on the edge. The latter is equally unlikely, as far as the prime minister sees it. Common taxes urged for eurozone Cameron's euro summit dilemma

He said there was a danger that saving the euro might be a case of "saving the cancer, not the patient".

And Northern Ireland Secretary Owen Paterson told The Spectator magazine that if there was a new eurozone bloc created "there will have to be" a referendum "because the pressure would build up".

Earlier, Downing Street said any treaty signed by the UK "will need to go through Parliament". It has said that a referendum will not be necessary because the proposed changes would not involve a big shift in power from London to Brussels.

Asked if Mr Paterson's comments made his position untenable, the prime minister's spokesman said no - because the government had a "clear policy" that only treaties which transferred power would result in a referendum.

Deputy Prime Minister Nick Clegg backed up Mr Cameron, saying the position on holding a referendum was clear.

"Of course there should be a referendum if we as a country were to surrender new powers to the European Union," he said.

"But there's no question of us doing that now because the new powers that will be surrended by anyone will be within the eurozone and of course we are not a members of the eurozone so the question simply does not arise"

In a joint letter on Wednesday, France's Nicolas Sarkozy and Germany's Angela Merkel called for the 17 eurozone countries to have common corporation and financial transaction taxes. They want a new treaty in place by March.

The government opposes any transactions tax that applies to the City of London but the PM's spokesman said, if it was limited to the 17 eurozone countries: "It certainly doesn't seem obvious to me that that is a threat to the City of London."

At Prime Minister's Questions, Mr Cameron faced questions from a stream of Conservative MPs who attacked any further integration in Europe.

John Baron said the PM should seize what he said was "a once-in-a-lifetime opportunity" to oppose further political union, while Andrew Tyrie warned against the risk of "a calculated assault" by the eurozone against the UK's position as Europe's leading financial centre.

Andrew Rosindell urged the PM to "show bulldog spirit" in a "resolute and uncompromising defence of British national interests".

If all 27 EU members were asked to sign a new treaty, Mr Cameron said the UK would expect to "get a price" in return. Should the members of the eurozone decide to pursue agreement on their own, the PM said the UK would still be able to exert "some leverage".

Media playback is unsupported on your device Media caption Boris Johnson: "If there is a EU new treaty that would lead to fiscal union...we'd have no choice either to veto it or put it to a referendum"

He said he would seek specific safeguards to give the UK "more power and control" in areas such as the single market and financial regulation.

"The more the countries in the eurozone ask for, the more we will ask for in return," he told MPs.

"The British national interest absolutely means that we need to help resolve this crisis in the eurozone... resolving this crisis is about jobs, growth, business and investment right here in the UK. At the same time we must seek safeguards for Britain."

'A handbagging'

But Labour leader Ed Miliband said the prime minister had promised to bring powers back to the UK in order "to quell a rebellion" by his backbenchers, but now could not name a single area where this might happen.

"Six weeks ago he was promising his backbenchers a handbagging for Europe now he is just reduced to hand-wringing. That is the reality for this prime minister.

"The problem for Britain is at the most important European summit for a generation, which matters hugely for families and businesses up and down the country, the prime minister is simply left on the sidelines."

If he lets this go on too long without there being a referendum it will wreck and destroy his government Nigel Farage, UK Independence Party

More than 80 Tory MPs defied the government last month and called for a referendum on the UK's membership.

Chris Heaton-Harris, founder of the Fresh Start group of MPs seeking reform of UK-EU relations rather than outright withdrawal, told the Daily Politics Mr Cameron should not do anything to delay a speedy solution to the eurozone crisis.

"I'm not convinced it (the summit) is going to be as bad as lots of media commentators are making out," he said. "I think there's a very good chance the Germans and the French will be helping us help them."

But Nigel Farage, leader of the UK Independence Party, which campaigns for the UK to leave the EU, told the BBC he did not believe the prime minister as his "policy on Europe keeps changing".

"If there is a new treaty and he signs up to it, it is clear it will have implications for us and there should be a referendum. If there is not a referendum, then the implications are not just a split within the Conservative Party but an increasing number of voters coming to UKIP."

He added: "If he lets this go on too long without there being a referendum it will wreck and destroy his government."