EU leaders were bracing themselves Thursday for a long night of negotiations with Britain over a reform package aimed at convincing the country to remain a member of the bloc.
"This is a make or break summit," EU President Donald Tusk said ahead of the two-day talks in Brussels. "We are in the middle of still very difficult and sensitive negotiations," he added.
Fears are rife that Britain - the world's fifth largest economy - might decide to leave the European Union in a referendum that Prime Minister David Cameron has promised to hold by the end of 2017, but that is widely expected this year already.
Many worry that Brexit - the buzzword for a British departure from the EU after more than 40 years of half-hearted membership - would reduce the bloc's global influence at a time when it is struggling with a migration crisis and enduring economic woes.
"The reaction [to Brexit] would surely be a wave of nationalism and separatism washing over all of Europe," Czech Prime Minister Bohuslav Sobotka told domestic lawmakers on Thursday, according to the CTK news agency.
German Chancellor Angela Merkel said keeping Britain in the EU is a matter of "national interest" for Berlin.
But with Cameron demanding reforms in the areas of sovereignty, immigration, economic governance and competitiveness, the EU's 27 other member states have been clear that Britain's continued membership must not come at any price.
"Poland wants to work on a solution that gives Britain a good position in the EU," the country's minister for Europe, Konrad Szymanski, told the TVN24 broadcaster. "But that does not mean that we will agree to any deal."
One of the crunch issues is a British demand to curb welfare payments to European workers, with a row brewing in particular over the issue of child benefits to Europeans whose families live outside Britain.
"We will accept no agreement directed against those who don't only pay taxes [in Britain], but have done so for many years," Szymanski warned.
Other unresolved issues relate to a British exemption to the EU's stated goal of "ever closer union," as well as a proposed mechanism for non-eurozone countries such as Britain to raise concerns about initiatives taken by the currency bloc.
Jeroen Dijsselbloem, the head of the Eurogroup of eurozone finance ministers, played down London's fears of being overruled by the currency bloc, pointing out that everyone is facing the same challenges, from migration to banking problems.
"We will respect their interests, but for a large part – certainly where the financial sector is concerned – we have a joint interest here," he told EU lawmakers.
Despite the unresolved issues, European Commission President Jean-Claude Juncker was optimistic Thursday of reaching a deal.
"It is not a done deal yet, but will be at the end of the day," he said, while adding that there is still a "considerable need for discussion" on some issues.
A British government source said there has been "a lot of progress," while noting that there is still work to do on "important, difficult issues."
Speculation has been rife that if a deal is reached, it will pave the way for Cameron to call the referendum for June 23.
It is hoped that the vote will end more than three years of uncertainty since Cameron first announced his intention to hold a referendum on membership in early 2013.