Divisions at the top of the Conservative Party were laid bare as Michael Gove clashed with Downing Street by claiming that 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.
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.
And the Government's senior law officer, Attorney General Jeremy Wright QC, said: "The suggestion that this agreement does not have legal effect until it is incorporated into EU treaties is not correct.
"It has legal effect from the point the UK says it intends to remain in the EU, and the European Court must take it into account.
"The job of the European Court is to interpret the agreements between the 28 nation states of the EU. This is one of those agreements, with equivalent legal force to other agreements such as treaties.
"That is not just my opinion - it is the opinion of this Government's lawyers, lawyers for the EU, and, I suspect, the majority of lawyers in this country."
The row came as Number 10 attempted to shift debate about EU membership to the impact on security - but admitted a blunder in a letter signed by former military chiefs.
The 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.
But a Downing Street spokesman admitted that the letter to The Telegraph had not been signed by former special forces chief General Sir Michael Rose.
"Due to an administrative error on our part, General Sir Michael Rose hadn't signed the letter which appeared in the Telegraph this morning," a spokeswoman said.
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 that Mr Cameron, 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".
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.
But he played down the prospect of the welfare curbs achieved by Mr Cameron having an impact on the number of EU citizens heading to the UK.
Asked if welfare curbs would deter migrants from moving to Britain, he replied: "I don't believe so."