Eurosceptic hopes that Boris Johnson will join the 'Brexit' campaign have risen after claims that he is "leaning towards" the UK quitting the EU.
Despite a Downing Street meeting with David Cameron on Wednesday, the Mayor of London has refused to change his belief that the decision is "finely balanced", Huff Post UK understands.
After the No.10 meeting, it appeared there was little sense of progress on key issues of Britain's sovereignty over EU laws or on the details of the negotiation with other EU leaders.
Boris set out his 'red lines' in his Telegraph newspaper column last month, covering border control, migrant benefits and 'red cards' to stop fresh EU legislation.
ITV’s Political Editor Robert Peston tonight claimed that Mr Johnson and Justice Secretary Michael Gove were both “leaning towards” backing the Leave campaigns.
Mr Johnson’s meeting with the Prime Minister did not get off to the best start, with the Tory MP kept waiting outside the famous 10 Downing Street door.
The meeting itself appears to have offered no significant reassurances to the London Mayor.
It is not just the London Mayor who Mr Cameron has to convince of the merits of the deal, but the heads of the 27 other EU states.
It had been expected that an updated version of the draft deal agreed with European Council President Donald Tusk would be agreed this evening ahead of the commencement of a summit tomorrow.
However, as of 11pm, no such update had arrived.
A Downing Street spokesman confirmed Mr Cameron and Mr Tusk had spoken on the phone "to take stock of progress ahead of tomorrow’s European Council."
The spokesman said: “The Prime Minister thanked President Tusk for the European Council’s hard work.
"They agreed that good progress had been made in all four areas of our renegotiation, and that the draft texts presented a good basis for agreement at tomorrow's European Council, subject to the satisfactory resolution of outstanding issues.
"They looked forward to continuing the discussion at their meeting ahead of the Council tomorrow afternoon.”
If Mr Cameron is able to secure agreement on the draft deal with other the EU leaders, a special meeting of the Cabinet is set to take place on Friday evening where collective ministerial responsibility over the issue is likely to be suspended.
This would enable those ministers backing Brexit –suspected to include Work and Pensions Secretary Iain Duncan Smith and Employment Minister Priti Patel – to begin campaigning for the Leave camps ahead of a likely referendum date of June 23.