David Cameron's deal changing Britain's relationship with the European Union is not legally binding and could be overturned by a prominent European court, Michael Gove has said.
The Justice Secretary rejected the Prime Minister's claim that the package was irreversible, warning that the European Court of Justice is not bound by the settlement without treaty change.
Downing Street moved quickly to dismiss Mr Gove's claim with a statement insisting the agreement "has legal force" and must be taken into account by the court.
As Mr Gove made his first major intervention in the referendum campaign since backing Brexit, his wife told of the "agonising" struggle he faced as he put his Euroscepticism ahead of a close friendship with the Prime Minister.
The Cabinet minister insisted the EU has held Britain back and said the nation would recover its "mojo" outside the 28-member bloc.
He insisted the Prime Minister, who has claimed the deal is "already legally binding and irreversible", has not misled voters.
But Mr Gove told the BBC: "The facts are that the European Court of Justice is not bound by this agreement until treaties are changed and we don't know when that will be.
"He's absolutely right that this is a deal between 28 nations, all of whom believe it. But the whole point about the European Court of Justice is that it stands above the nation states.
"The Prime Minister is right, this is a deal that those 28 nations stand behind, I don't believe that he has been misleading anyone.
"I do think it's important that people also recognise that the European Court of Justice stands above every nation state, and ultimately it will decide on the basis of the treaties and this deal is not yet in the treaties."
Mr Gove said the failure of the single currency and problems with migration showed the EU was an "old-fashioned model".
"I think it would be a tremendous opportunity for Britain to recover its mojo, for Britain to be a more flexible, outward looking, creative place. One of the big problems with the European Union is that it has held us back."
He added: "The nature of the European Union is bureaucracy. The failure of the single currency, the problems that it has had with migration, all of them point to the fact that it is an old-fashioned model. It's sclerotic, it's out of date."
His wife, journalist Sarah Vine, said in her Daily Mail column that her husband had been "locked in an internal struggle of agonising proportions" over his decision on the referendum, but had chosen "own heartfelt beliefs" over "loyalty to his old friend, the Prime Minister".
Downing Street said the deal secured at a marathon summit in Brussels was "irreversible".
A Number 10 spokesman said: "It is not true that this deal is not legally binding. Britain's new settlement in the EU has legal force and is an irreversible international law decision that requires the European Court of Justice to take it into account."
Martin Schulz, president of the European Parliament, told the BBC that exempting Britain from ever closer union would be "written exactly literally like it is" in the deal in future treaty change.
Asked if welfare curbs would deter migrants from moving to Britain, he replied: "I don't believe so."
He added: "For the first time since a long time a Conservative Prime Minister of the United Kingdom is fighting for Europe and for the European Union. This is a progress in itself."
Meanwhile, a group of former senior military commanders have warned that leaving the European Union could hamper the UK's ability to tackle threats such as Islamic State or Vladimir Putin's Russia.
In a letter to the Daily Telegraph, the ex-services chiefs told of their concern about the rise of IS, also known as Daesh, saying: "We are proud to have served our country and to have played our part in keeping Britain safe. In the forthcoming referendum, therefore, we are particularly concerned with one central question: will Britain be safer inside the EU or outside it? When we look at the world today, there seems to us only one answer.
"Europe today is facing a series of grave security challenges, from instability in the Middle East and the rise of Daesh, to resurgent Russian nationalism and aggression.
"Britain will have to confront these challenges, whether it is inside or outside the EU. But within the EU, we are stronger. Inside it, we can continue to collaborate closely with our European allies, just as we did when we helped to force the Iranians to the negotiating table through EU-wide sanctions, or made sure that Putin would pay a price for his aggression in Ukraine."
Signatories to the letter, which was in part co-ordinated by Downing Street, include former chiefs of defence staff Field Marshal Lord Bramall and Field Marshal Lord Guthrie.
Air Chief Marshal Lord Stirrup, Admiral of the Fleet Lord Boyce and former special forces chief General Sir Michael Rose are also among the 13 senior officers who backed the letter.