Mr Cameron has promised the UK vote on leaving or remaining in the EU will take place before the end of 2017
European Council President Donald Tusk has walked out of crunch talks with David Cameron on Britain's future in Europe less than two hours after arriving in Downing Street.
As he left Number 10, Mr Tusk told media there was "no deal".
His early exit marks a stark contrast to remarks he made entering the building, when he said he was hopeful of a deal.
It throws the Prime Minister's hopes of holding a referendum on EU membership this summer into serious doubt.
Mr Cameron tweeted shortly after Mr Tusk left Downing Street - and described the talks as a "good meeting".
He said Mr Tusk had "agreed to another 24 hours of talks before publishing the draft UK renegotiation text".
The PM wanted powers to stop EU migrants claiming benefits for four years after they enter the UK.
Mr Tusk, though, had offered an "emergency brake" - giving Mr Cameron his demands if he can prove the UK economy is suffering.
However, this compromise is not supported in Eastern Europe.
A breakthrough tonight was crucial because an EU summit on 18-19 February has to finalise the deal.
Without that, there will be no UK referendum this summer.
Sky's Chief Political Correspondent Jon Craig, in Downing Street, said: "Donald Tusk has rather dramatically left here after less than two hours and told Sky News 'no deal'.
"How much of the menu they got through, I don’t know."
Mr Cameron has promised the UK vote will take place before the end of 2017.
But eurosceptics say Mr Cameron faces an uphill struggle to ratify his proposals for reform.
Co-chair of Conservatives for Britain, Steve Baker, told Sky News: "Vote Leave (the campaign group that wants Britain to leave the EU) has shown that nine out of 10 of the PM's reform proposals have been dropped.
"It is not likely we will get child benefit reform that was planned and in particular we know that if anything is achieved, it will be because the policy applies equally to all. This is a fundamental problem.
"Everything the Prime Minister does is that it has to be on the basis of EU citizenship rather than British citizenship."