As Prime Minister Theresa May braces for a new battle with MPs over Brexit, one man stands at the centre of it all with the power to shape what happens next.

House of Commons Speaker John Bercow is best known for presiding over debates, bellowing "Order! Order!" at unruly MPs and chastising ministers who displease him.

But today he will play a decisive role in whether Mrs May can stick to her plan to leave the European Union on 29 March, or whether MPs will be allowed to try to stop her.

Worryingly for the government, he has shown he is happy to rip up the rule book to allow parliament to have its say.

Referee of our affairs

John Bercow was first elected as an MP for the Conservative party, but his elevation to Speaker in 1999 means that he is supposed to be politically neutral.

However, his interventions in Brexit debates have prompted accusations from his former colleagues that he is biased both against the government and Brexit itself.

Opposition Labour MPs instead broadly support him.

He has also been accused of presiding over a culture of bullying in the Commons, and did not deny he referred to a senior female government minister as "stupid".

Exchanges between MPs and the Speaker have become increasingly bad-tempered but tensions reached boiling point earlier this month.

Defying precedent and the advice of his clerks, Mr Bercow allowed MPs to amend a government motion on Mrs May's Brexit deal.

The effect was to force Mrs May to quickly return to the Commons to explain herself if the agreement was rejected by MPs - as it was a few days later.

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What next for Brexit? 

Mr Bercow's decision sparked an angry row in the chamber, as Conservative MPs and ministers stood up to denounce him to his face.

"Many of us will now have an unshakeable conviction that the referee of our affairs... is no longer neutral," one said.

Another challenged him over a sticker spotted in the car he drives, saying "Bollocks to Brexit".

Mr Bercow replied tersely that it belonged to his wife and "she is entitled to her views", and insisted he was only standing up for the rights of individual MPs.

Today, he will be back in the driving seat when MPs debate May's efforts to salvage her divorce agreement.

Frustrated by the lack of progress, they will try to force her hand through amendments demanding she seek a new plan, delay Brexit or call a new referendum.

It falls to Bercow to decide ahead of the debate which of these amendments will be put to a vote.

Bullying claims

Born in 1963 into a modest family, John Bercow grew up in London and was a child tennis champion, leading to a lifelong love of the sport.

John Bercow reacts during the ATP World Tour Finals tennis tournament in London in 2018

He was always a Conservative, but in his youth held right-wing views that he has now rejected.

He became an MP in 1997 and 12 years later was elected as Speaker, becoming the youngest person to hold the role for 100 years.

Mr Bercow has sought to modernise parliament, abandoning the Speaker's traditional robes for a simple gown over a suit, and seeking to make it easier for women MPs with new babies.

John Bercow in ceremonial garb at the opening of parliament in 2017

But critics say he is pompous, and over-fond of the sound of his own voice - one Conservative MP famously described him as a "stupid, sanctimonious dwarf".

Before the recent Brexit clashes, he also enraged many Conservatives with his outspoken opposition to allowing US President Donald Trump to address parliament.

He is due to retire in the next year, but there have been suggestions he could stay on to see the Brexit process through - an idea likely to dismay the government.

Given his support from the opposition, ministers can do little about it although reports suggest they might deny him a seat in the House of Lords when he steps down as punishment.