David Cameron will step up efforts to finalise the details of his EU reform package amid signs Brexit campaigners are struggling to find a "big beast" to lead them.
The Prime Minister will meet key players in the process, including the presidents of the European Council and European Parliament, as he continues work to secure a deal.
Mr Cameron will meet EU counterparts on the margins of a London summit about the Syria crisis, before hosting European Parliament president Martin Schulz at Downing Street.
He will also meet Donald Tusk, the European Council president, who published the draft deal on Tuesday.
The tricky task still facing Mr Cameron has been underlined by suggestions that MEPs could delay introduction of a so-called "emergency brake" on migrant benefits for 18 months.
Former home secretary Alan Johnson, the leader of Labour's campaign to remain in the EU, backed the benefit curbs but said they would do nothing to reduce immigration.
The Prime Minister was given a rough ride by a series of Tory backbenchers in the Commons on Wednesday, with Jacob Rees-Mogg complaining they were being offered "thin gruel".
Mr Cameron acknowledged further work will be needed to secure reform in an "intense" round of negotiations ahead of the February 18 EU summit.
Brexit campaigners continued to struggle to resolve their differences - both internally and between the two rival groups - as former chancellor Lord Lawson was announced as a key player.
Vote Leave and Leave.EU are engaged in a bitter battle to be designated as the official Brexit campaign by the Electoral Commission.
Vote Leave, which has been suffering a bout of internal squabbling, has announced a shake-up of personnel in a bid to draw a line under the problems.
Tory former cabinet minister Lord Lawson has been installed as chairman, replacing businessman John Mills, while campaign director Dominic Cummings and chief executive Matthew Elliott have stepped down from the organisation's board in a move a spokesman claimed was "previously planned".
Arron Banks, founder of Leave.EU, suggested the changes could have removed the obstacles to a merger between the bodies.
But Tory MP Steve Baker, chairman of the Conservatives for Britain group, told BBC2's Newsnight that was "impossible".
"There are no plans for a merger. I believe Vote Leave will win the designation, I believe Vote Leave will win the referendum," he said.
"I don't think it is possible. The reason is there are genuine disagreements over strategy and tactics."
Mr Baker said he would be happy to go into the referendum campaign with the team of politicians who are already backing Brexit, likening them to Shakespeare's "happy few".
However, he admitted he wanted to see more sign up, joking that his "dream" was for Mr Cameron to lead the Leave side.
Asked about reports that London mayor Boris Johnson was ready to support Remain, he said people "shouldn't believe everything they read".