The British Prime Minister's Brexit plans were thrown into further turmoil when the speaker of parliament ruled that she could not put her divorce deal to a new vote unless it was re-submitted in fundamentally different form.
In comments that blindsided Theresa May's office, Speaker John Bercow said the government could not bring forward proposals for a vote in parliament that were substantially the same as had been defeated twice before, in January and last week.
Brexiteers seeking a complete break from the European Union saw a "no-deal" exit as now more likely, but the government made clear it would seek to put off Brexit beyond the 29 March departure date, if the EU approves.
One of the government's senior law officers, Solicitor General Robert Buckland, said: "We're in a major constitutional crisis here."
He told the BBC one way to bring Mrs May's deal back for a vote in the House of Commons could be prorogation - ending the parliament session prematurely and starting a new one.
According to precedents stretching back to 1604, parliamentary rules say that substantially similar proposals cannot be voted on in the House of Commons more than once during the same session of parliament.
Mr Bercow said his ruling should not be considered his last word and the government could bring forward a new proposition that was not the same as those already voted upon.
The pound fell to its day's low against the euro and the dollar on Mr Bercow's statement, before recovering when the government said negotiations on a deal were continuing with the DUP, who prop up Mrs May's minority government and have opposed her withdrawal accord so far.
"This is my conclusion: if the government wishes to bring forward a new proposition that is neither the same, nor substantially the same, as that disposed of by the House on the 12th of March, this would be entirely in order," Mr Bercow said.
"What the government cannot legitimately do is to resubmit to the House the same proposition or substantially the same proposition as that of last week which was rejected by 149 votes."
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The ruling was welcomed by eurosceptic MPs in Mrs May's Conservative Party because it appeared to increase the likelihood of Britain leaving the EU without a deal.
Mrs May's Withdrawal Agreement negotiated with the EU last year was seen by Brexiteers as leaving Britain too closely aligned to the EU while depriving it of voting rights in the bloc.
"May I say how delighted I am that you have decided to follow precedent, which is something I am greatly in favour of," said Jacob Rees-Mogg, chairman of the European Research Group of eurosceptics in parliament.
However Kwasi Kwarteng, a Brexit minister, told parliament the government intended to seek an extension to the Brexit departure deadline, which he expected the EU to decide on at a summit this week.
The head of the 2016 referendum campaign to leave the EU, Matthew Elliott, said he expected MPs to "see sense" and pass Mrs May's deal by 29 March.
Mr Bercow's pronouncement appeared to take May's Downing Street office by surprise.
A spokesman said her office had not been warned the statement was coming. Nor could Downing Street say anything about plans for a new Brexit vote or when it might be held.
Tánaiste and Minister for Foreign Affairs Simon Coveney said that there was surprise at what Mr Bercow said in the House of Commons.
Speaking to RTÉ News in Brussels, he said: "A think a number of us listening to Westminster today were somewhat surprised that a procedure of the House [of Commons] has been used to potentially prevent a meaningful vote happening before the summit.
"But that is what it is, and that’s a matter for Westminster to resolve."
EU leaders have ruled out renegotiating the exit deal.
Earlier, it had appeared that Mrs May was winning support for her deal from erstwhile opponents but Mr Bercow's decision will leave the prime minister scrambling for options.
Asked by an MP whether further changes to the deal would be needed, Mr Bercow said, "in all likelihood, the answer ...is yes", adding that a change in opinion would not constitute a change in the deal on offer.
"Fundamentally, for something to be different, it has to be by definition, fundamentally different. Not different in terms of wording, but difference in terms of substance and this is in the context of a negotiation with others outside the United Kingdom," he said.
Britain is due to leave the EU by default in 11 days.
However, parliament voted last week for a delay and Mrs May is expected to seek an extension to that deadline when she meets EU leaders at their summit.
To compound her problems, Mrs May appeared unlikely to reach agreement this week on her Brexit plans with the DUP.
Her spokesman said talks were continuing with the party.
She had earlier warned lawmakers that unless they approve her Brexit divorce blueprint, Britain's exit from the EU could face a long delay which many Brexiteers fear would mean Britain may never leave.
After two-and-a-half years of negotiations with the EU, the outcome remains uncertain - with options including a long postponement, exiting with Mrs May's deal, a economically disruptive exit without a deal, or even another EU membership referendum.
Mrs May's blueprint, an attempt to retain close trading and security ties with the EU while leaving the bloc's formal political structures, was defeated by 230 votes in parliament on 15 January, and by 149 votes on 12 March.