Up to five Cabinet ministers will campaign for Britain to leave the European Union and David Cameron should allow them to speak out now, the former defence secretary has said.
Eurosceptic Liam Fox warned David Cameron risked creating a lasting split in the Conservative party if he suppressed opposition to the EU until after any deal was done.
He told the BBC's 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."
He added: "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.
"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."
European Commission president Jean-Claude Juncker has said the EU reform proposal is fair for both Britain and for the other 27 EU member states.
He told the assembly in Strasburg the proposal must be supported by the European Parliament.
I have always said I want the UK to remain a member of the European Union on the basis of a fair deal.
The settlement that has been proposed is fair for the UK and fair for the other 27 members.
It is also fair for the European Parliament ... the parliament must be fully signed up to the new settlements.
The proposals for an emergency brake and a red card would damage the EU's ability to respond to a crisis, a Dutch MEP has said.
Sophia in 't Veld, vice-chairwoman of the Alliance of Liberals and Democrats for Europe group, told BBC Radio 4's Today programme: "Much of what is in this deal is already in the treaties, it is a matter of interpretation.
"Parts of the package will have to be voted by parliament, there will be a regular legislative proposal on the table - we don't know what that will look like so we can't say."
But Belgian MEP Sander Loones, from the New Flemish Alliance, said the emergency brake was needed.
Mr Loones said: "My plea is for, as Mr Cameron says, a strong European network, not a strong European superstate. We want a Europe that is more competitive. That also means that we do need these emergency brakes."
He added: "A lot of common sense is now lacking in this European Union. We are squeezed between federalist Germany, between interventionist France. What we do need is the free market view, the common sense view of the UK. This is what this package brings to the table."
David Cameron will seek to persuade Conservative MPs to back his EU reform proposals when he appears in the Commons on Wednesday.Read the full story ›
David Cameron will have to persuade many of his own backbenchers and some members of his Cabinet to support the EU reforms outlined on Tuesday.
Despite campaigning not officially beginning until after the deal is approved, some firmly-held views have already been exchanged.
ITV News Political Correspondent Libby Wiener reports:
Home Secretary Theresa May has described the EU proposals unveiled on Tuesday as "a basis for a deal".
Mrs May - who had been tipped as a possible leader of the "out" campaign - said more work needed to be done on the plan set out by European Council president Donald Tusk.
In a statement, Mrs May said: "EU free movement rules have been abused for too long and EU law has stopped us deporting dangerous foreign criminals.
"That is plainly wrong and it is encouraging that the commission has agreed with the UK that we should take action to address these two issues.
"So we have made progress and negotiations continue ahead of the February council. As the Prime Minister has said, more work needs to be done, but this is a basis for a deal."
Members of the European Parliament have been reacting to the draft proposals for reform of Britain's EU membership.Read the full story ›
Draft proposals for reform of Britain's EU membership were published on Tuesday, but what are the main points included in the package?Read the full story ›
Euro-sceptic MPs within the Conservative Party say that the draft reforms announced by Donald Tusk do not go far enough.Read the full story ›