The European Council President says it is a "crucial moment for the unity of our Union"
There is "still no guarantee" that a new EU deal for Britain will be reached at the crucial Brussels summit, Donald Tusk has warned.
The European Council President told EU leaders there would be difficulties during talks on Thursday and Friday, and that failure "would be a defeat for the UK and the European Union".
In a letter, he called it a "crucial moment for the unity of our Union".
He said: "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."
Mr Tusk said there was "not be a better time for a compromise" and warned deadlock would be a "geopolitical victory for those who seek to divide us".
The final draft of the proposed shake-up is expected later.
David Cameron is still trying to galvanise support for the deal before EU leaders decide whether to green-light the changes.
Proposals include a four-year 'staggered' ban on migrants' in-work benefits and changes to how much child support they get.
If a deal is done, it will trigger frenzied campaigning ahead of an expected 23rd June referendum.