LONDON (Reuters) - Once Britain triggers its formal divorce from the European Union by invoking Article 50 of the Lisbon Treaty, there is no going back on the decision, the government’s top lawyer told the High Court on Monday.

A European Union flag is waved outside the High Court in Central London, October 17, 2016. REUTERS/Stefan Wermuth

As Britain moves towards exiting the world’s biggest trading bloc after June’s vote that shocked financial markets, there is debate over how one of the most complex negotiations in recent history will work. London’s view that Article 50 cannot be reversed is also at odds with recent statements from Brussels.

The government is facing a legal challenge by claimants who say that only parliament can decide when, how and whether to trigger the formal divorce by invoking Article 50.

Prime Minister Theresa May’s government says that it has the right to decide when to start the divorce and that Article 50 will be invoked by the end of March next year, ushering in a two-year negotiating period to decide Britain’s exit terms.

At a hearing, Attorney General Jeremy Wright told the High Court that a notification invoking Article 50 - probably with a letter from May - was irrevocable.

“We do not argue that an Article 50 notice can be revoked, and we invite the court to proceed in this case on the basis that a notification ... is irrevocable.”

Asked by Lord Chief Justice John Thomas, Britain’s most senior judge, if that meant the notice could not be conditional, Wright said: “We accept that.”

His guidance appears to resolve one of the biggest questions of the Brexit negotiations: Whether Britain could at some point decide to revoke its intention to leave after negotiations begin.

Article 50 of the EU’s 2009 Lisbon Treaty, which was drafted by a former British ambassador to the EU, has never been used, providing no legal precedent for how it works.

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Donald Tusk, the European Council president who will oversee the Brexit negotiations, said last week that Britain was free to simply withdraw unilaterally its proposal to leave at any point before the two-year notice period ends.

May, who has repeatedly said that “Brexit means Brexit” and that she will make a success of the move, suggested earlier this month that those who say parliament should first approve triggering the divorce were “not standing up for democracy, they’re trying to subvert it”.

Investment manager Gina Miller, the lead claimant in the case, said she had received death threats, and Justice Thomas warned those responsible they would face the “full vigour of the law”.

“It is simply wholly wrong for people to be abusive of those who seek to come to the queen’s court,” he said.

LEGAL CHALLENGE

The argument for parliamentary approval before invoking Article 50 is based on an analysis of the authority for EU law in the United Kingdom. Under this reasoning, EU law applies in Britain because of the European Communities Act of 1972, and the executive cannot undermine rights given by parliament to the people of Britain by triggering Article 50.

Government lawyers dispute that and say the executive alone has the right to notify the EU of Britain’s intention to leave. The government has rejected a petition signed by more than four million people demanding a second referendum.

Wright said there was an established principle that governments had the right to make or withdraw from international treaties using the ancient power known as royal prerogative.

He said the argument from the claimants was not a narrow legal challenge and suggested they were seeking to overturn the referendum result.

“It seeks to invalidate the decision already taken to withdraw from the EU and to require for that decision to be taken by parliament,” Wright said.

A verdict in the case is expected by the end of the month. It is likely that the matter will be appealed to the British Supreme Court for a final decision before the end of the year.

If the court challenge is successful, the government could be forced to pass legislation authorizing Article 50 to be triggered, requiring a vote by lawmakers, the majority of whom voted to Britain to remain in the EU.