EU President Donald Tusk says he cannot guarantee that a summit starting on Thursday will deliver a deal on reforms that Britain has demanded to remain a member of 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," Tusk writes in a letter to EU leaders.
"We differ on some political issues and I am fully aware that it will be difficult to overcome them. Therefore I urge you to remain constructive," he adds.
Tusk says the summit will be "a crucial moment for the unity of our union," warning that a loss of unity "would be a defeat both for the UK and the European Union, but a geopolitical victory for those who seek to divide us."