The House of Commons Speaker has thwarted any attempt by Theresa May to bring a third meaningful vote to parliament, unless there has been substantial change to the Brexit deal.

With Theresa May’s plans thrown into chaos by the move, one of her chief law officers warned the government could be forced to cut short the parliamentary session and restart in order to bring back the Brexit deal.

John Bercow’s shock move, which drew immediate criticism from May’s allies, suggested he believed such a fundamental change would involve a renegotiation at EU level rather than clarification of the legal advice written by the attorney general, something that had been suggested this week.

The solicitor general, Robert Buckland, said the decision was a “constitutional crisis” and that the government might have to consider the drastic step of ending the parliamentary session early and restarting a new session.

John Bercow has seized his Brexit moment – and now all bets are off | Rafael Behr Read more

“We’re in a major constitutional crisis here ... This has given us quite a lot to think about in the immediate term. There are ways around this – a prorogation of parliament and a new session – but we are now talking about not just days but hours to 29 March,” Buckland told BBC News.

“Frankly we could have done without this, but it’s something we’re going to have to negotiate with and deal with.”

Downing Street was blindsided by the announcement and unable to give a response at its regular afternoon briefing for journalists. “The speaker did not forewarn us of the content of his statement or the fact that he was making one,” May’s spokeswoman said.

Ministers described the government as being in a state of shock. “It’s miserable, I think the first thing is the government having to come to terms with it,” one said.

Q&A What is the role of the Speaker and how are they elected? Show Hide The Speaker is the chief officer and the highest authority in the House of Commons. They chair parliamentary debate to call MPs to speak, to keep order, and to instruct when votes should be taken.

The Speaker is expected to remain politically neutral on all issues, and to continue this even in retirement. On appointment as Speaker, they resign from their political party. They continue to stand for election as an MP, but by tradition they are unopposed in their constituency by the major parties. The Speaker has the power to compel MPs to withdraw remarks, to suspend individual MPs, or to suspend the whole sitting of the House of Commons in case of serious disruption. Speakers are elected by MPs in a secret ballot. MPs are given a list of candidates and make their choice. If a candidate receives more than 50% of the votes, the question is put to the house that he or she takes the chair as Speaker. If no candidate reaches 50%, the candidate with the fewest votes and those with less than 5% of the vote are eliminated, and MPs vote again until one candidate succeeds. Sir Lindsay Hoyle was elected Speaker on 4 November 2019, replacing John Bercow, who stepped down from the role having been elected as Speaker in June 2009. Photograph: PRU/AFP

Bercow made the surprise announcement in the chamber on Monday afternoon, saying the Commons was “being repeatedly asked to pronounce” on the same question.

Quoting from the guide to parliamentary procedure, Erskine May, Bercow said by convention, the question “may not be brought forward again during the same session” and that it was a “strong and longstanding convention” dating back to 1604.

He said the convention had been confirmed again many times, including in 1864, 1870, 1882, 1891 and 1912. “Indeed, Erskine May makes reference to no fewer than 12 such rulings up to the year 1920,” he said.

“One of the reasons the rule has lasted so long is that it is a necessary rule to ensure the sensible use of the house’s time and the proper respect for the decisions it takes.

“Decisions of the house matter. They have weight,” he said. “It is a necessary rule to ensure the sensible use of the house’s time and the proper respect for the decisions which it takes.”

Bercow said the second meaningful vote motion held last week did not fall foul of the convention, because it “could credibly be argued it was a different proposition” to that rejected on 15 January because of changes the government considered to be legally binding.

“If the government was to bring forward a new proposition that is neither the same, nor substantially the same as disposed of by 12 March, this would be entirely in order,” he said, but added that it could not be “the same proposition or substantially the same proposition”.

In a response to a point of order by the chair of the Brexit select committee, Hilary Benn, Bercow said a “demonstrable change to the proposition would be desired … a change in opinion about something does not in itself constitute a change of the offer”.

He said the change must make the deal “fundamentally different”, rather than just a unilateral reinterpretation.

“Not different in terms of wording, but different in terms of substance,” he said. “This is in the context of a negotiation with others, outside the United Kingdom. That would be my initial feeling.”

Q&A What does the Speaker's ruling on a third meaningful vote mean? Show Hide Speaker of the House of Commons John Bercow ruled on 18 March that he would not allow parliament another vote on the same Brexit deal. Prime minister Theresa May would have to make 'substantial' changes to her government's two previous attempted to pass the withdrawal agreement deal with the EU. Bercow said: "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 March 12, this would be entirely in order. What the government cannot legitimately do is resubmit to the house the same proposition - or substantially the same proposition - as that of last week, which was rejected by 149 votes. This ruling should not be regarded as my last word on the subject. It is simply meant to indicate the test which the government must meet in order for me to rule that a third meaningful vote can legitimately be held in this parliamentary session." Photograph: Reuters Tv/X00514

Bercow won immediate support from the Conservative Brexiter Bill Cash, in a flurry of points of order after the statement. “It seems to me that what you have said makes an enormous amount of sense given the fact that, actually, this has been defeated on two separate occasions,” Cash said.

The Speaker confirmed the motion could be put forward unchanged if parliament was prorogued and then resumed, in answer to a question from the pro-Brexit Conservative MP Jacob Rees-Mogg.

There is a part-precedence for such a prorogation in the passage of the 1949 Parliament Act, which reduced the powers of the House of Lords in delaying certain legislation. The law was blocked by the Lords twice, over two parliamentary sessions.

Since the existing law which the new act was replacing – the 1911 Parliament Act – required three parliamentary sessions to pass before the Commons could overturn the Lords, the Attlee government prorogued parliament – ending the session – and began a new special session lasting from 14-26 September 1948, complete with its own King’s speech.

The Labour MP Stephen Doughty asked Bercow if this could happen again, to which the Speaker replied it would be “an unusual step”.