Prime Minister Theresa May has said she will invoke Article 50 by the end of March, kicking off two years of divorce negotiations.
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"You can change your mind while the process is going on," John Kerr, a former British ambassador to the EU who drafted Article 50, told the BBC.
"During that period, if a country were to decide actually we don't want to leave after all, everybody would be very cross about it being a waste of time," he said. "They might try to extract a political price but legally they couldn't insist that you leave."
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Attorney General Jeremy Wright told the High Court last month that a notification invoking Article 50 - probably with a letter from May - was irrevocable.
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