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".

Former defence secretary Liam Fox has suggested up to five Cabinet ministers could back Brexit in the looming referendum - now regarded as almost certain to be held on June 23.

They are thought to include Work and Pensions Secretary Iain Duncan Smith, Leader of the House Chris Grayling and Northern Ireland Secretary Theresa Villiers.

But No 10 will have been heartened by the apparent unwillingness of Mr Johnson and Home Secretary Theresa May to throw their weight behind Leave.

Justice Secretary Michael Gove and Foreign Secretary Philip Hammond also look set to fall into line behind Mr Cameron.

Eurosceptics have called for Cabinet ministers in favour of Brexit to be able to speak out now, rather than wait for the conclusion of a deal on the reforms.

But Cabinet minister Justine Greening defended the approach, set out in rules issued by Mr Cameron, which prevents her colleagues adopting a stance at odds to the Prime Minister until the campaign begins in earnest.

She told BBC Radio 4's Today programme: "I happen to agree with the Prime Minister, I think this is a good deal, I hope we can seal the deal when he goes to Brussels later this month.

"But in the meantime we have Cabinet collective responsibility and indeed the deal isn't finally agreed yet. So I think we all need to back the Prime Minister to get the best deal for our country, that is in everyone's interests. Let's see what he finally manages to come up with - I think he has made good progress - and then we will have the referendum.

"In the end it won't just be for Cabinet ministers to say where they stand on this, it will be for everyone in our country."

She added: "I would like us to be able to stay in a Europe where we have a better, renegotiated deal. For me it's about interest and influence - it's in our interest because it matters for jobs and it's about influence because the discussions that happen around the table at the European Union are absolutely relevant to our lives day-to-day here in Britain and we should be at that table if we possibly can be and get a good deal."