Bercow, 56, is the most controversial Speaker of the House of Commons in recent memory -- and he’s a crucial figure influencing the UK’s divorce from the European Union. He voted to remain in the EU in the 2016 referendum and has a reputation for giving rank-and-file members of Parliament time and influence during key debates.

Brexit has thrown up all kinds of obscure questions of Parliamentary procedure on which the Speaker must make the final ruling. Since May lost her Commons majority in 2017, and now runs a weak minority government, ordinary members of Parliament and the Speaker have considerable power to shape policies.

Most dramatically, on Monday Bercow derailed May’s plan to put her unpopular agreement for the U.K.’s split from the EU back to members of Parliament for a third vote. They have rejected it overwhelmingly twice already.

Bercow invoked a rule dating back to 1604 under which the House of Commons should not be asked to make a decision on the same question twice. If May’s deal is no different from the one Parliament threw out last week by a huge 149-vote majority, she can’t put it to another vote, he said.

The likely outcome is that May will now have to ask for a long delay to Brexit, lasting perhaps more than a year, when she meets fellow EU leaders in Brussels at a summit on Thursday.

May’s team privately accuse Bercow of trying to force Britain into a lengthy extension to the negotiating period in order to allow Parliament to take over and impose a softer form of Brexit. They say he has a history of trying to frustrate the U.K’s departure from the EU.

While Bercow is technically a member of May’s Conservative Party, he is required to be politically impartial at all times, giving fair time to opposition politicians. He is not permitted to vote except in extremely rare cases.

His relations with May’s office have long been strained amid accusations that he’s biased against the party he once championed. But Brexit has brought them to a new low.

The Speaker has a pivotal role in Parliament, shaping debates, ordering politicians to stop speaking, and keeping proceedings in what can be a rowdy and unruly Commons chamber running smoothly. He has the power to select amendments, and rule on which motions are in order for the House to consider. Amendments have been one of the key tools that members of Parliament have used as they try to shape the divorce.

Politicians who are trying to soften Brexit see Bercow as an ally. The opposition Labour member of Parliament Hilary Benn urged him last year to take care when crossing roads.