Prime Minister David Cameron arrived at the EU summit tonight insisting he will be battling for eurozone stability and the protection of British interests.
He said eurozone stability was good for the whole of Europe, including the UK, adding: "We've got to protect British interests and that's what we will be discussing."
Before leaving London he said he would have "no hesitation" in wielding Britain's veto to block an EU treaty change to resolve the eurozone crisis if it did not meet UK requirements.
Mr Cameron is coming under intense pressure from some elements of his own party to demand the return of powers from Brussels to Westminster as the price for acquiescence in a treaty, and to put any deal to the British people in a referendum.
Downing Street insists that no referendum is needed, as proposed changes will not transfer powers to Brussels, and Mr Cameron has said that he will ensure that any treaty includes safeguards to protect the City of London from European regulation.
Mr Cameron spoke briefly as he went into the start of what could turn out to be marathon negotiations - the eighth EU summit on the eurozone crisis this year.
He said: "These are important talks and we need to get that stability in the eurozone. It's good for Europe and good for Britain. We've got to protect British interests and that's what we will be discussing."
His first meeting before a dinner with all 26 other EU leaders was due to be a private chat with Italian Prime Minister Mario Monti, brought in as a technocratic caretaker leader after the fall of Silvio Berlusconi.
Mr Monti announced fresh austerity measures for Italy at the start of the week in what was seen as part of a coordinated series of moves designed to build up market confidence in the run-up to a landmark deal at the summit itself.
French President Nicolas Sarkozy and German Chancellor Angela Merkel today called for swift action when they addressed members of the centre-right European People's Party in Marseille ahead of the summit opening this evening.
They have made clear that if they cannot secure support for a treaty change from all 27 EU members in Brussels, they are ready to press ahead with an accord limited to the 17 countries which use the single currency.
As well as tough new controls on eurozone economies, including automatic sanctions if eurozone states breach debt and deficit criteria, the pair are calling for a tax on financial transactions within the zone, common corporate tax rates and harmonised employment rules.
Mr Sarkozy warned: "The longer we delay, the more costly and less productive the solution will be. If we don't have an agreement on Friday, there won't be a second chance."
None of the Franco-German proposals would apply to the UK, which is one of 10 EU states outside the eurozone.
The leader of Mr Cameron's Tory MEPs, Martin Callanan, tonight joined the chorus calling for a referendum.
He said: "If there is any treaty change which creates European fiscal union then clearly that will radically effect the UK and that should be put to a referendum.
"That is what democracy demands, because we would be creating a fundamental change to the EU and our relationship with it."
He went on: "However, that could take years to complete. It might be a way to solve the next crisis - but not this one.
"That is why the focus should be on measures to address the issues at the heart of this crisis."
Mr Callanan criticised what he described as the "casual one-size fits-all approach" which had characterised the single currency policy from the start.
He said the eurozone area reflected a massive economic imbalance between its "prosperous and economically-disciplined" members and others which were "debt-ridden and financially dysfunctional".
Mr Callanan warned: "We should be recognising that the eurozone needs to be smaller to survive. It needs to lose some members. That will cause enormous pain, and we are talking about the least-worst option, but as a start, Greece needs to default, devalue and depart the euro.
"This is what the summit should be tackling instead of navel-gazing about treaty changes and playing pass the parcel with a pile of debt."
The German leader of the Socialist Group in the European Parliament, Martin Schulz, said: "A debate about treaty change will not help countries currently under pressure to regain the confidence of financial markets.
"I am sceptical about changing the Lisbon Treaty. European citizens are not interested in years of debate about EU structures and institutions. They want decisive actions to stabilise the crisis now."
In a dig at Mr Cameron, he warned: "If there is too much insistence on treaty change there is a danger of backroom deals and trade-offs, which we do not want to see. I want clarity and transparency.
"I understand those who refuse to engage in horse-trading with a non-euro country. But we cannot allow the division of Europe into a 17-10 structure. Maybe, we should think about a 26-1 solution."
He attacked an eve-of-summit announcement by credit rating agency Standard and Poor's which suggested that the triple-A rating of the whole of the EU may be downgraded because of the eurozone crisis, saying: "The agencies currently escape legal responsibility for their actions because their ratings are considered to be opinions.
"We want them to be defined as services so that they come under the same rules as products or services, with the same need to be responsible in law.
"The job of these agencies is to evaluate risks for their clients, not to make political judgments. The timing of Standard and Poor's announcement - a few days before a European summit - is highly questionable."
Labour leader Ed Miliband described the summit as "one of the most important summits in a generation" - but said Mr Cameron seemed "incredibly weak because he's leading a divided Government and a divided party".
Speaking during a visit to Tioga, a contract electronic manufacturer in Derby, he said: "I think the problem is that the Prime Minister looks more isolated and lacking in influence than a prime minister has looked for a long, long time in a European summit and frankly that's because of the way he's handled it."
Mr Miliband went on: "First of all, saying it was nothing to do with him; then saying he was going to bring back powers to Britain and use treaty change to win back powers from Brussels; then conceding that wasn't going to happen; then threatening a veto and, frankly, I think, most of his European partners seem to be shrugging their shoulders and saying 'Well, look, he's not interested in positively engaging to get a good outcome here, he's interested in just shouting from the sidelines and trying to satisfy his own backbenchers'.
"And that's why I think Britain so sorely lacks influence at this summit."
Ahead of the summit dinner, Mr Cameron held private talks with Italian Prime Minister Mario Monti, and with Chancellor Merkel and President Sarkozy.
The Prime Minister made clear that, if there has to be treaty change, British interests must be safeguarded.