LONDON — Just when you thought Brexit couldn't get any more complex or arcane, it now looks like the crucial next 10 days will be shaped by decisions made in 1604.

That's because of a colossal constitutional wrench thrown into the works by the speaker of the House of Commons, John Bercow, who has sent an already chaotic process spiraling deeper into uncertainty.

Unlike the speaker of the House of Representatives in Washington, the British speaker is supposed to be an impartial figure who chairs debates between lawmakers in Parliament.

However on Monday, Bercow caused uproar by telling Prime Minister Theresa May that she was not allowed to bring her Brexit deal back for a third vote unless it contained significant changes.

John Bercow makes his statement to the House of Commons on Monday. Parliamentary Recording Unit / AFP - Getty Images

Justifying this political thunderbolt, Bercow quoted an 1844 reference book written by Erskine May, the theorist and clerk of the House of Commons. This document has been updated to more than 1,000 pages and continues to be the de facto bible for parliamentary procedure.

The speaker went on to quote parliamentary convention dating back to April 2, 1604 — three years before English settlers first arrived in Jamestown, Virginia.

"It is a bit weird. That's the year before Guy Fawkes tried to blow the place up in the gunpowder plot, so it is certainly going back a bit," said Jill Rutter, program director at the Institute for Government, a think tank, referring to a foiled conspiracy to bomb the Houses of Parliament. "But when you have a Parliament that's been around a long time, that's what you'd expect."

On Monday night, to the delight of parliamentary history geeks, "Erskine May," "1604," and "Bercow" were all trending on U.K. Twitter.

Why does this matter?

The United Kingdom voted to leave the E.U. in a referendum in June 2016. But since then, politicians have failed to agree on a plan that translates this yes-no question into reality.

The U.K. and Europe are linked by more than four decades of shared laws and regulations, operating almost like a single country in many respects. Brexit negotiations have gotten stuck on how best to unpick this relationship — deciding which parts to keep and which to ditch.

Few thought thrashing something out would take this long. And now it really is crunch time.

At the moment, U.K. law says that its membership of the E.U. will end on March 29, at 11 p.m., whether there's a deal in place or not. Most experts say a "no-deal Brexit" would be an unprecedented act of economic self-harm.

The prime minister has negotiated a divorce settlement with the E.U. that would allow the U.K. to uncouple and start negotiating a future relationship. However, while May has managed to reach a deal with European officials, most U.K. lawmakers have staunchly opposed it — for a wide range of reasons.

Her deal has been crushed twice in the House of Commons — suffering the heaviest and fourth heaviest losses in parliamentary history. Much of this opposition has come from May's own Conservative lawmakers, and in December some in her party attempted a coup to topple her.

And yet — at what feels like the 11th hour and 57th minute of Brexit — May and her deal are somehow still in the game.

This obstinacy led Dutch Prime Minister Mark Rutte to compare her to the Black Knight in "Monty Python and the Holy Grail," who refuses to admit defeat despite having of all his limbs chopped off in a sword fight.

"She's incredible. She goes on and on," Rutte told Dutch television Sunday.

This week, May was gearing up for a third-time-lucky attempt to push her deal through Parliament, hoping that the lack of time and fear of the alternatives would bring enough naysayers onside.

Enter Speaker Bercow.

Pointing to these centuries-old conventions, the speaker said that because May's deal was pretty much the same thing lawmakers had rejected twice already, she could not ask them to keep voting on the same thing again and again until she won.

"It is the parliamentary equivalent of the double jeopardy rule," the government's former director of legislative affairs, Nikki da Costa, wrote in Britain's Daily Telegraph on Monday.

Although he was a Conservative Member of Parliament before he became speaker, Bercow is deeply unpopular among many in his former party. He is often criticized as someone more interested in grandstanding and self-promotion rather than chairing debate.

"I think the idea that we could have got through another week in the Brexit psychodrama without Bercow trying to put himself at the center of it is probably an unrealistic expectation," said Rutter, at the Institute for Government.

Lawmakers have long since gotten used to raucous parliamentary debates being punctuated by Bercow bellowing "Order!" above the din. And in one of his colorful interventions Monday he called out one senior lawmaker, Leader of the House Andrea Leadsom, for "playing with her electronic device" during the debate.

Tomorrow's front page: Speaker John Bercow torpedoed Theresa May's EU deal - and stuck two fingers up to 17.4million Brexit voters https://t.co/bfFKbaLEtK pic.twitter.com/Wi7zfvgLmh — The Sun (@TheSun) March 18, 2019

No fewer than eight British newspapers featured Bercow on their front pages Tuesday — few of them flatteringly.

"What Bercow would say is that this a government that's trying to game its unloved deal through Parliament by wearing them down and making them keep voting until they just give in," Rutter added. "He would say that you aren't supposed to be able to bully Parliament into submission."

Many Remainer and Brexiteer opponents of May's deal supported the speaker's intervention. Both camps believe that by decreasing the chances of the prime minister's deal passing, it increases the chances of the different outcomes that they favor respectively.

So what now?

Bercow's intervention means that all eyes will be on a pivotal summit of the European Council on Thursday.

British lawmakers have already instructed their prime minister to ask these European leaders to extend the current Brexit deadline of March 29.

Because May is now unlikely to pass her deal this week, the extension Europe offers could be one of months or even years. Many experts thought May would try to use the fear of this long extension to bring pro-Brexit opponents of her deal into line.

Her likely plan was to present these lawmakers with a choice: Either support my imperfect deal or open up the risk of never leaving the E.U. at all.

This strategy has clearly been frustrated by Bercow's intervention. The government is reportedly now scrambling to find a way to get around his ruling.

One way might be to convince the E.U. to tweak the deal just enough to satisfy Bercow's demands.

A more drastic option would be something called a "prorogation of Parliament" — essentially a brief and technical break that means any third attempt to pass the deal would not be in the same session, and therefore circumventing Bercow's ruling.

Another option might be to press the nuclear button and call for a general election.

Although this would allow the opposition Labour Party the chance to win power, it might be the only way to reshuffle the parliamentary arithmetic if the government simply can't make the current numbers work.