Britain's tortuous road to exit the European Union has taken another dizzying turn. The Speaker of the House of Commons on Monday seized on a 415-year-old precedent to deny Prime Minister Theresa May a third vote on her Brexit deal. Mr John Bercow ruled that Mrs May needs to present a substantively different version of her plan from the one that has been decisively voted down twice, by 230 votes and 149. A remedy for this constitutional crisis that now obstructs the delivery of Brexit is for her to press for a vote again after she meets the EU today. She is likely to secure backing for an extension of three months, possibly much longer, to the March 29 deadline by which Britain must part ways with the EU.

Whether Mr Bercow acquiesces that a new deadline counts as a substantial revision, worthy of a vote, and whether the House then backs the deal are the next two hurdles for Mrs May. Thereafter looms an even stiffer challenge - a demand gaining ground that the deal passed by the House be approved by a "confirmatory referendum" through which voters will assess whether the government has indeed designed a desirable Brexit.