European Union (EU) heads of state on Thursday began a two-day summit meeting in order to try to reach a deal for Britain to remain in the bloc.
"After my consultations in the last hours, I have to state frankly: There is still no guarantee that we will reach an agreement," European Council President Donald Tusk said on the eve of the summit.
"We differ on some political issues and I am fully aware that it will be difficult to overcome them."
Tusk also urged EU leaders to "remain constructive" during negotiations with British Prime Minister David Cameron, EFE news reported.
The member states are discussing British demands to stay in the EU, before it will hold a referendum that is suggested to be held in summer on whether to leave the bloc.
"There will not be a better time for a compromise. It is our unity that gives us strength and we must not lose this," Tusk said.
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The summit aims to achieve a legally binding agreement that addresses Britain's concerns, and satisfies, at the same time, all other members.
The leaders will work on the basis of Tusk's draft on deal for Britain presented on February 2, which included key four points for a new settlement of the country within the EU, which included four items: economic governance, competitiveness, sovereignty and social benefits and freedom of movement.