LONDON -- After the deed was done, some leaders didn't want to shake his hand. French President Nicolas Sarkozy walked right by him, as if he wasn't there.

David Cameron, the British prime minister, had become Europe's outcast.

His sin? Rejecting an invitation to join 26 European partners in a tighter financial alliance to save the euro, making Britain odd man out at a time of deep financial peril, and raising doubts about whether Britain can realistically remain a member of the European Union.

Former British Europe minister Denis MacShane, a House of Commons legislator and longtime advocate of closer ties, said the sudden break with the other 26 countries means Britain's role in the European Union is effectively over.

"There is now little point in Britain staying in the EU," said MacShane, who was a minister in Tony Blair's generally pro-Europe Labour Party government. "It is an historic turning point and Britain might as well get out now, as Europe's future will be settled without us."

On the other side of the political spectrum, Robert Oulds, director of the euroskeptic think tank the Bruges Group, agreed.

"Britain being part of the EU but holding up plans that everyone else wants is just a cause of friction," he said. "It's creating trouble for David Cameron at home. There's no middle way which can appease British public opinion and his own party and the interests of the European political class that wants more integration."

His prescription: a referendum on Britain's "amicable" exit from the EU.

That would be a momentous step.

The European Union, which has grown from an alliance of six nations in 1951, has been a cornerstone of Europe's strategy to keep the peace since World War II by bringing former adversaries together through common economic and cultural ties.

Unprecedented prosperity has flowed from its single market and its free movement of workers across borders. Integration accelerated in the past two decades with the development of a single currency, but now the euro's survival is under threat as the continent's debt woes spin out of control.

The EU is today so intertwined that even member countries like Britain that do not use the euro would plunge into deep and prolonged recession if the currency broke up. That's a sobering reality that helped persuade 26 of 27 member states to consider big sacrifices of national sovereignty to keep the European dream alive.

For Britain, an island nation that prides itself on its fierce independence, the idea of ceding any more control over its destiny proved too much.

Cameron, exhausted after an all-night session and with his silk tie uncharacteristically askew, insisted Friday afternoon that Britain remains an influential EU member.

"We want to be in Europe for trade, investment, jobs and growth," he said, adding that Britain is a leading member of the NATO alliance.

But he wanted no part of the new treaty, which he said didn't adequately protect Britain's national interest and meant giving up too much control over regulation of Britain's dominant financial sector.

Cameron's desire to remain part of the EU while refusing to join the new treaty angered European leaders, who don't want Britain to cherrypick what aspects of EU life it is willing to embrace.
Italian Prime Minister Mario Monti said that Britain would no longer be "in the heart of Europe" as it was during the Blair years.

"I think a certain isolation will come," he said, predicting that Britain will still be at the table but that its "capacity to influence" events will be greatly diminished.

Cameron has warned as recently as last month about the dangers of leaving the EU. He said in a November speech that withdrawing would jeopardize Britain's vital trade ties and leave Britain subject to European regulation but unable to help draft them.

"Believe me, if we weren't in there helping write the rules they would be written without us -- the biggest supporter of open markets and free trade -- and we wouldn't like the outcome," he said.

The economic impact of leaving the European Union would be difficult to predict. British companies might lose easy access to European markets -- where they now enjoy open trade, with few barriers -- but Britain also might be able to negotiate a favorable trade treaty with Europe, as Israel and Mexico have done.

Although some tabloid newspapers routinely mock EU regulations, Britain has thrived on attracting talent from other EU nations under rules that allow workers from other member nations to find work in Britain without serious bureaucratic hurdles. Open borders have helped make London an ever more international city.

No country has ever left the EU. Greenland got out in 1982, but as a Danish dependency it wasn't a full member at the time.

Britain joined the European Economic Community -- as the EU was then called -- in 1973.

Harold Wilson's Labour Party government held a referendum on membership in 1975, and two-thirds of voters approved.

But that enthusiasm has faded with time.

Britain's relations with the EU were at their most rancorous during Margaret Thatcher's time as prime minister between 1979 and 1990. One of her most famous remarks on the EU was simply, "No! No! No!"

Thatcher was particularly alarmed by the ambitions of Jacques Delors, who as president of the European Commission was working to create a centralized European government.

"We have not successfully rolled back the frontiers of the state in Britain, only to see them re-imposed at a European level with a European super-state exercising a new dominance from Brussels," she said in a 1988 speech.

In October, 81 Conservative Party lawmakers challenged Cameron to hold a referendum on continuing in the EU -- a vote that was not binding on the government, but represented the largest rebellion so far against the leader from within his own ranks.

A referendum would be potentially explosive to the prime minister because he leads a coalition government in conjunction with the Liberal Democrats, the most openly pro-European of Britain's major parties.

Mary Honeyball, an opposition Labour Party lawmaker who represents London at the European Parliament, said there would almost certainly be a domestic fallout for Cameron's coalition, especially since Deputy Prime Minister Nick Clegg, the Liberal Democrat leader, has advocated closer European ties.

"Nick Clegg is, as we know, very pro-European and yet he is also part of a government which has loosened ties with the EU and lost the respect of European heads of state," she said. "This will undoubtedly have implications for the survival of the coalition."

Clegg said Friday he backed Cameron's decision, but would "continue to argue within government" in support of European ties.