Former defence secretary Liam Fox has said he is “certain” four or five serving Cabinet ministers speak out in favour of Brexit.
He also warned of a lasting split in the Tory party if the prime minister suppresses opposition until after a deal is done.
David Cameron appeared to receive a major boost this week when Home Secretary Theresa May – previously tipped as a possible leader of the Out campaign – said the Tusk proposals provided “a basis for a deal”.
But London Mayor Boris Johnson – who has said the prime minister is “making the best of a bad job” – has yet to ally himself with either side, fuelling speculation eurosceptics may now look to him to spearhead the drive for the UK to leave.
In the Commons, Mr Cameron, who faces calls to allow Cabinet colleagues to start campaigning for withdrawal from the EU now, told Conservative MPs to “vote with their hearts” at the in-out referendum.
He also said they should not vote a certain way if they “think it might be advantageous” – in what may be interpreted as a reference to the ambitions of his potential successors.
Meanwhile, European Commission President Jean-Claude Juncker has hailed the draft deal “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”.
Former foreign secretary David Miliband also entered the fray yesterday, warning Brexit would not solve the refugee crisis – and could make it worse.
Now president of the International Rescue Committee, he said leaving could lead to a “very, very rocky period” for Britain.