CNN  — 

The Speaker of Britain’s House of Commons has thrown a constitutional curve ball into the Brexit process.

Citing a convention that dates back to 1604, John Bercow thwarted an attempt by Prime Minister Theresa May to hold another vote on her beleaguered Brexit bill this week.

Parliament has already rejected it twice by substantial margins. But with a deadline of March 29 approaching – at which the UK crashes out of the EU unless an agreement is reached – May’s government hoped to bring the deal back a third time. There were even reports that if it failed again, she might return for a fourth attempt.

But Bercow ruled that, according to parliamentary procedure, the government could not repeatedly put a motion before lawmakers if it had been previously rejected in the same session.

May’s deal suffered a second, crushing defeat last week when it was rejected by 149 votes.

“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,” Bercow said in an unannounced statement on Monday.

Bercow said that, in his view, the first two motions on May’s Brexit deal were sufficiently different not to have broken parliamentary convention.

The speaker did not set out what tests the government would have to meet if it was to successfully submit its deal for a third vote, saying only it would have to be “fundamentally different.”

In response, a Downing Street spokesperson said, “We note the speaker’s statement. This is a statement that requires proper consideration.”

UK political commentators have speculated that changes to the political declaration associated with the deal could be enough to meet the speaker’s requirements. It has even been suggested that the government could ask the Queen to prorogue Parliament and start a new session to get around his ruling.

Solicitor General for England and Wales Robert Buckland told the BBC that “we’re in a major constitutional crisis” and that Bercow’s ruling has “given us a lot to think about.”

May already faced an uphill task to get her deal past parliament, requiring 75 MPs to change their minds. In an effort to get her deal over the line, Cabinet ministers spent the weekend in talks with the Northern Irish Democratic Union Party, or DUP. The DUP’s 10 MPs, who prop up May’s minority government, have so far opposed the deal. It is thought that if they switch sides, Brexiteer Conservatives will follow.

It seems unlikely the Prime Minister will attempt to hold a new vote on her deal before a European Council summit in Brussels at the end of this week.

After rejecting May’s Brexit deal last week, MPs voted in favor of an extension to the withdrawal process, given the unlikelihood of agreeing a deal before March 29. May is expected to ask the remaining 27 EU member states for a delay at this week’s summit.

It’s possible the EU may propose a long extension to the Brexit process and require the UK to take part in the upcoming European elections in May.

Downing Street has used the prospect of a lengthy delay – which could be used to force a second referendum – to try to persuade Brexiteer lawmakers that they risk losing Brexit altogether if they don’t vote for May’s deal.

CNN’s Luke McGee and Jane Merrick contributed to this report.