Europe woke up on Friday to the cold certainty of an unprecedented British referendum after David Cameron's election win, with two years of tough negotiations ahead to prevent a so-called "Brexit".
Work was to start quickly at the European Union's headquarters in Brussels after Cameron, whose Conservatives look set for a surprise majority, confirmed his intention to let Britons vote in 2017 on membership of the bloc.
But while the rest of Europe has said they won't keep Britain in the club at any price, the unexpected result could increase Cameron's negotiating power and make it more likely that Britain stays part of the EU, analysts and officials said.
"The risk of Brexit is strongly reduced today, because Cameron has just received a full mandate and his position in his party has been strengthened," a senior European diplomat in Brussels told AFP.
Mats Persson, director of the Open Europe think-tank, said that after months of "posturing" Europe would now take the referendum seriously because it was a "new event, we have never seen something like that before. It's huge event for the EU."
"It's now getting real for people," Persson told AFP.
In his victory speech on Friday after an election that will apparently end five years of coalition with the europhile Liberal Democrats, Cameron said the new government "must hold" a referendum "to decide Britain's future in Europe."
Cameron has vowed to renegotiate London's relationship with Brussels and then go to the electorate, saying he wants Britain to remain in a reformed EU.
Friday's result in Britain is the starting gun for what are likely to be bruising negotiations over the coming two years, beginning with a European summit in June at which Cameron is likely to set out the reforms he wants.
A top EU official told AFP however that the referendum "is winnable."
"We have to wait to see what he wants to negotiate. We certainly have work to do," the official said, adding: "I think that will start pretty quickly".
The mood music from Brussels has been more melodious in recent weeks, with European Commission chief Jean-Claude Juncker last week opening the door to minor changes to the EU's treaties.
With the British election coming as much of Europe celebrated the 70th anniversary of Victory in Europe Day (VE Day), the theme of European unity was much on the minds of many in Brussels.
But Juncker, along with most EU leaders, have rejected out of hand any fundamental changes such as limiting the free movement of people inside the EU, which Cameron has demanded.
"The British are wrong if they think their partners are ready to pay a heavy price to keep them in, especially if that price deprives the whole European project of any meaning," another European diplomat told AFP in Brussels.
However there was an increasing feeling in Brussels on Friday that Cameron's strong majority at home could smooth his path and thereby help keep Britain in the EU.
"The most important outcome of UK elections is that Cameron could now be a strong leader of his own party, which is key for the coming nasty Brexit debate," said Jan Techau, director of the Carnegie Europe think-tank.
"Mr Cameron will claim that he has a strong mandate. But he needs to come out clearly with what he wants from Europe," Techau told AFP.
The failure of Nigel Farage and his anti-EU UK Independence Party in the election -- Farage did not win a parliamentary seat -- made it easier for Cameron to sell EU membership to the British public, diplomats said.
"Cameron can now organise issues how he likes around the economic advantages of the EU, which he understands," the first European diplomat said.