David Cameron is making final efforts to bolster support for his proposed European Union reforms before a crunch summit.
As fraught negotiations continued in Brussels over the details of the package, Boris Johnson was among senior figures called to Downing Street for talks with the Prime Minister.
The mayor of London is yet to make clear which side he will back in the in/out referendum on Britain's membership of the 28-nation bloc, with a senior source saying he remains "genuinely conflicted".
He interrupted a half-term break for around 40 minutes of discussions with the PM, with Brexit campaigners still hopeful he will provide a high-profile focus for the Leave campaign.
"I'll be back, no deal," Mr Johnson, who spoke to the PM by telephone on Tuesday, said to waiting reporters as he left.
He is not expected to make his position public until a final agreement has been reached between EU leaders.
Mr Cameron is spending the day at Number 10 as he prepares to meet fellow leaders on Thursday for talks on the UK's reform demands. Agreement at the summit would open the door to a public vote within weeks.
He has been warned he has an "extra mile" to go to persuade eastern European leaders to agree to reforms, notably over curbs to benefits for EU workers.
European Council president Donald Tusk is expected to publish the final draft of the proposed shake-up later, with leaders scheduled to discuss them on Thursday evening and into Friday morning.
If he secures a deal Mr Cameron will call a Cabinet meeting, effectively firing the starting gun on the referendum battle, as Eurosceptic ministers will then be free to campaign for a Leave vote in the poll expected on June 23.
Downing Street conceded there "are still details to be nailed down" to secure an agreement this week but insisted Mr Cameron's talks with key figures in Brussels on Tuesday had been "useful".