David Cameron's bid to win backing for his EU reforms has reached "crunch time", a UK government official says.
The prime minister heads for a Brussels summit later hoping to agree a deal on changes that will pave the way for the UK's in/out referendum.
It follows months of negotiations between officials, with the PM visiting 20 member states to make his case.
EU Council President Donald Tusk told the BBC leaders had "no choice" but to agree a deal at the two-day summit.
Campaigners for a UK exit from the EU have dismissed the draft reforms as trivial, saying the only way to achieve change is to vote to leave.
In his official invitation to the gathering, Mr Tusk said there was "no guarantee" a deal would be reached, with "difficult" differences remaining on key issues.
He said the negotiations were at a "very advanced" stage and failure now "would be a defeat both for the UK and the European Union, but a geopolitical victory for those who seek to divide us".
Following a phone call between Mr Cameron and Mr Tusk late on Wednesday, a Downing Street spokesman said they had "agreed that good progress had been made... and that the draft texts presented a good basis for agreement at tomorrow's European Council, subject to the satisfactory resolution of outstanding issues".
