David Cameron is making final efforts to bolster support for his proposed European Union reforms but has been warned there is "no guarantee" he will reach an agreement.
The Prime Minister will join fellow EU leaders in Brussels for crunch talks on his demands, but European Council president Donald Tusk said there were issues which would be "difficult to overcome".
But Mr Cameron was boosted by Angela Merkel's declaration that most of his demands were "justified and necessary", while French prime minister Manuel Valls said it would be a "shock" for Europe if the UK voted to leave the EU.
In a letter to EU leaders, Mr Tusk said the summit on Thursday and Friday "will be a crucial moment for the unity of our union and for the future of the United Kingdom's relations within Europe".
He said: "After my consultations in the last hours I have to state frankly: there is still no guarantee that we will reach an agreement. We differ on some political issues and I am fully aware that it will be difficult to overcome them."
But "there will not be a better time for a compromise", he said, adding: "It is our unity that gives us strength and we must not lose this. It would be a defeat both for the UK and the European Union, but a geopolitical victory for those who seek to divide us."
Mr Tusk said the aim was to reach a "legally binding and irreversible agreement which addresses UK concerns, while being satisfactory to all" and he hoped a deal would be done at the summit.