LONDON (Reuters) - London’s High Court said on Tuesday it would rule “as quickly as possible” on whether British MPs, and not the government alone, must trigger the formal process of leaving the European Union, in a case closely watched by politicians and markets.

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Campaigners have taken legal action to argue Prime Minister Theresa May and her ministers do not have the authority to invoke Article 50 of the EU Lisbon Treaty, the mechanism by which a nation can leave the bloc, without the explicit backing of parliament.

During three days of legal wrangling involving some of the country’s top lawyers, the High Court has heard from the government that a decision to trigger Article 50 could not be reversed and that it was “very likely” the final exit deal agreed with the EU would have to be ratified by parliament.

However, the key issue of whether May can invoke Article 50 using the ancient power of “royal prerogative” remains to be determined.

“We shall take time to consider the matter and will give our judgements as quickly as possible,” said Lord Chief Justice John Thomas, Britain’s most senior judge who has been hearing the case with two other leading justices.

Whichever side loses will almost certainly appeal to the Supreme Court, the UK’s highest judicial body, which will give a final verdict in December.

May has said she would invoke Article 50 by the end of March next year, starting a two-year divorce procedure but promised that lawmakers would be fully consulted over the subsequent negotiations.

Sterling has fallen to 31-year lows since May’s announcement and the court case is being closely monitored by market players who believe the greater parliament’s involvement, the greater the chance there is of a “soft Brexit”, where Britain stays in or remains close to the EU single market.

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Lawyers for the government argued it was established constitutional convention for the executive to make or withdraw from international treaties using prerogative powers.

The Attorney General, the government’s top lawyer, has also told the court that once invoked, the process to leave the EU is irrevocable, although Donald Tusk, the European Council president who will oversee Brexit talks, said last week Britain was free to simply withdraw unilaterally its proposal to leave at any point before the two-year notice period ends.

On Tuesday, another government lawyer James Eadie said it was probable parliament would have to ratify the final exit deal, words which helped buoy the pound.

“The government view at the moment is it is very likely that any such agreement will be subject to ratification,” Eadie said.

That would mean lawmakers could potentially block the deal, although Eadie said it was possible that the EU and Britain could agree that the divorce agreement would come into effect without parliamentary approval.

Lawyers representing those who want parliament and not May to invoke Article 50 said this was all far too late anyway.

“If parliament refuses its agreement, we still leave the EU,” said David Pannick, the lawyer representing the lead claimant in the challenge. “By the time parliament comes to look at the matter post-notification, the die is cast,”