EU leaders will attempt next week to thrash out a reform deal with Britain, in a bid to prevent a "Brexit," which some fear could start unravelling the European Union.
Britain has long been a reluctant member of the 28-country EU, wresting special arrangements from the bloc and challenging integration efforts down through the years.
Now, Prime Minister David Cameron has promised to hold a referendum on whether Britain should stay in the EU and said that he will only campaign for the country to remain a member if the bloc agrees to reforms on immigration, competitiveness, economic governance and sovereignty.
Fears are rife that a Brexit, as Britain's potential departure from the EU has come to be known, would destabilize the bloc politically and economically - and at worst could even start to tear it apart.
"This is not only a problem for the United Kingdom or for the relations between the EU and the UK. In fact, this is about the future of Europe," EU President Donald Tusk warned regional representatives in Brussels on Wednesday.
Tusk will embark next week on a whirlwind tour of five European capitals to sell a compromise reform package that he hopes will convince Britons to vote in favour of continued EU membership. However, other EU leaders must agree to it first.
Cameron's most controversial demand has been a request to restrict access to benefits for other European citizens working in Britain, an effort to limit the number of people seeking jobs in Britain. However, the free movement of workers is one of the EU's most cherished achievements.
As a compromise, Tusk has suggested a "safeguard mechanism" that would allow Britain to restrict access to in-work benefits for four years.
His other compromise proposals include a "red card" system to allow national parliaments to stop or amend EU legislation and a mechanism to address fears that further integration within the eurozone could harm British interests.
Tusk said he is confident that a consensus will be found on Britain's future in Europe, while British Chancellor of the Exchequer George Osborne told journalists on Friday that "we have the real prospect of agreeing a plan for a reformed EU."
But the leaders' negotiations at a summit in Brussels next week are nevertheless expected to be thorny.
Central and Eastern European countries, whose citizens frequently seek work elsewhere in the EU, have been particularly wary of Cameron's demands.
"We cannot agree with any changes to the current framework that would mean permanent barriers to free movement," Czech State Secretary Tomas Prouza wrote in an opinion piece on the EUobserver website. "We need to protect our core values and freedoms."
It also remains to be seen if Cameron will be won over by Tusk's proposals, which he welcomed as delivering "substantial change," but which have been criticized in Britain.
Cameron has promised to hold the referendum on EU membership by the end of 2017. Many analysts, however, expect him to do so already in June 2016 if a reform deal is struck at Thursday and Friday's summit.
Also on the leaders' agenda will be the EU's other major preoccupation: a migration crisis that saw more than 1 million people arrive in the bloc last year, including many refugees from Middle Eastern war zones.
The leaders will call for the rapid implementation of agreed response measures, which have been slow to materialize, and further action in the main migration transit countries of Turkey, Greece and Italy, according to a draft of their statement seen by dpa.
Turkish Prime Minister Ahmet Davutoglu is due in Brussels on Thursday and is expected to take part in pre-summit talks with some EU leaders, diplomats said.
The migration crisis is the "worst political context" for the British referendum, giving eurosceptics a handy tool to "show how vulnerable Europe is today" and to blame the EU, Tusk said this week.
The future of Europe's cherished free-travel Schengen area also hangs in the balance. Tusk has predicted that it could collapse if the EU does not get its migration crisis under control by mid-March.
"It is no exaggeration to say that the next six weeks will be key for the future of the EU," he said on Wednesday.