David Cameron has insisted "good progress" has been made on his EU negotiations on the eve of crunch talks with state leaders in Brussels.
The Prime Minister spoke with European Council president Donald Tusk ahead of the summit, where he hopes to lock down agreement of his demands which could spark the start of the referendum campaign.
A Downing Street spokeswoman said: "They agreed that good progress had been made in all four areas of our renegotiation, and that the draft texts presented a good basis for agreement at tomorrow's European Council, subject to the satisfactory resolution of outstanding issues.
"They looked forward to continuing the discussion at their meeting ahead of the Council tomorrow afternoon."
Mr Cameron was boosted on Wednesday by German chancellor 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.
But Mr Tusk warned that there is "no guarantee" he will reach an agreement and there were issues which would be "difficult to overcome".
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.