Boris Johnson has said that there is "a lot more to do" before he can back a package of proposed reforms to allow the UK to remain in the European Union.
The London Mayor was speaking as David Cameron prepared to address MPs on the proposals, which he said on Tuesday were "worth fighting for".
The Prime Minister faced calls for former defence secretary Liam Fox to allow Cabinet ministers to start campaigning openly for withdrawal from the EU.
Dr Fox predicted up to five Cabinet ministers will back Brexit in the upcoming referendum, and warned that the Prime Minister risks creating a lasting split in the Conservative Party if he tries to gag them any longer.
In Strasbourg, European Commission president Jean-Claude Juncker hailed the draft deal as "fair for the UK and fair for the other 27 member states".
But he risked antagonising Eurosceptics by saying that the euro "remains the currency of the EU", and that a proposed emergency brake on migrant benefits will be "limited in time" and would apply only in "exceptional circumstances".
The brake would help address a situation created by the UK Government's own failure to apply transitional restrictions more than a decade ago, he told the European Parliament.
After the draft deal drawn up with European Council president Donald Tusk received a broadly hostile reaction from the right-leaning press, Mr Cameron has the opportunity to gauge the response of his own MPs as he delivers a statement to the House of Commons.
Mr Johnson told Sky News the PM was "making the best of a bad job".
Ahead of the unveiling of Mr Tusk's package on Tuesday, the Mayor said he had "doubts" about a proposed red-card scheme to give national parliaments more power over Brussels laws.
Asked whether he had changed his mind after seeing the document, he replied: "I think the position is very much the same as it was yesterday morning, which is a lot more to do on this .. Let's wait and see when this whole thing is agreed and try to see what it really means - every bit of it."
Many supporters of Brexit see Mr Johnson as their favoured figurehead to spearhead the Out campaign in the in/out referendum, which is widely expected to be held in June.
Home Secretary Theresa May appeared to rule herself out of that role in a statement on Tuesday evening, saying that the Tusk package provided "a basis for a deal".
Dr Fox said Mrs May and other ministers would "make up their own minds", but told the BBC Radio 4 Today programme: "I think there will be a number of Cabinet ministers with a range of different responsibilities who will want to be in the Leave camp ... I don't know exactly how many, but I can think of four or five for certain."
Mr Cameron has told ministers he will suspend the principle of collective responsibility to allow them to campaign on either side of the debate, but not until a Cabinet meeting following the conclusion of any deal, which could come at a Brussels summit on February 18-19.
But Dr Fox suggested that the PM himself already appeared to be campaigning in favour of the deal in a speech he delivered on Tuesday in which he said the reforms offered Britain "the best of both worlds".
Chancellor George Osborne also appeared to give his backing to Mr Tusk's proposals, saying during a visit to Rome that they "both strengthen the European Union and strengthen Britain's position in the European Union and address some of the concerns Britain has had about the European Union, the result of which, I think, if we can get an agreement in the coming weeks, will lead to a stronger, more reformed Europe that Britain can be part of".
"If the Prime Minister has already decided that the draft deal is enough for him to campaign, to go out there selling the deal, then it should be for others who don't agree with that to make their own case," said Dr Fox.
"I think the danger of treating the two sides differently is that it will make it more difficult for us to come together after the referendum."
In Strasbourg, Eurosceptic MEPs were scathing about the package, with the DUP's Diane Dodds dismissing it as "a smoke-and-mirrors charade" and Independent Janice Atkinson as "a Chamberlain moment" - in an apparent reference to the doomed 1938 attempt to forge a peace deal with Hitler.
Ukip leader Nigel Farage said that "people power" and not politicians would decide the UK's future relations with Europe.
But the Conservative leader of the ECR grouping, Syed Kamall, insisted the deal was "a good place to start" in debating "matters that should benefit all European Union countries".
And the sole Liberal Democrat MEP Catherine Bearder said staying in the EU would be "better for our prosperity, our security and our environment".
Meanwhile, venerable Tory think tank the Bow Group warned Conservative MPs who campaign to remain in the EU could face deselection when seats are reallocated under planned boundary changes.
Pointing to a recent poll suggesting 71% of party members back Brexit, Bow Group chairman Ben Harris-Quinney said: "Conservative Associations will be unforgiving towards any MP who thinks they can say one thing in an election pledge, and do another when in Parliament."