The latest row erupted as it was reported the Attorney General Geoffrey Cox - who is due to make a statement to the Commons today - had warned the United Kingdom could be tied to the EU customs union "indefinitely" through the Northern Ireland "backstop". "But if she's lost a vote of this significance after two years of negotiation, then it is right that there should be a general election because, but for the Fixed-term Parliaments Act, the convention was always, if a government loses what's called a confidence vote - something of such significance - then that government has to go", Starmer said.
He admitted there were aspects of the deal he found "uncomfortable", but said his message to MPs was that they "should not make the ideal the enemy of the good".
The government believes a unilateral exit mechanism for the backstop would be impossible to negotiate - not least because the European Union would be concerned about what might then happen if May was replaced by another prime minister. "I know it is challenging", he told the BBC on Sunday.
The spat over the legal advice came after the government insisted on publishing only "a reasoned position statement", rather than the full advice, in line with the parliament's demands.
Michael Gove has insisted it is possible for the government to pass Theresa May's Brexit deal through Parliament, even though many MPs have threatened to vote against it.
The prime minister doubled down on her deal when quizzed by journalists in Argentina on Saturday.
"I believe that we can win the argument and win the vote".
The paper reported on a letter sent by Cox, in which the top law officer said: "The [backstop] protocol would endure indefinitely" - and claimed Britain would only be able to lose the backstop through a trade deal, which could take years to reach and on which the European Union would hold a veto. "They are essentially either we leave with no deal which would have serious economic consequences, or we say to the British public: 'I'm sorry you got it wrong, you're going to have to do it again, ' which I think would have serious democratic consequences". She said Britain would develop its own sat-nav system, at an estimated cost of several billion pounds (dollars).
"We have got to recognize that if we don't vote for this, the alternatives are no deal or no Brexit". "It's important to note that we are making him available", a No 10 source said.
He pointed out that "all parties" would press for contempt of Parliament proceedings if MPs are not shown the advice, the legal opinion of British Attorney General Geoffrey Cox.
Labour confirms it will submit a no confidence motion in the government if May loses the vote.
"With ministers warning a defeat could derail Brexit altogether, today a separate cross-party group of MPs calls for parliament to support an amendment for another referendum at the earliest opportunity".