The British government will need to put forward a parliamentary bill to trigger formal European Union divorce talks should it lose a Supreme Court fight over who can start the Brexit process, a government lawyer says.
However, this could be just a one-line piece of legislation, James Eadie added -- an option which could limit anti-Brexit lawmakers' ability to delay the process or water down government policies on the terms of the EU exit.
"It would require not just parliamentary involvement ... but primary legislation," Eadie told the Supreme Cour on Tuesdayt.
"The reason it requires primary legislation is (because) you are being asked to declare ... unlawful the exercise of the prerogative power to give Article 50 notice as the first step in the process," he told the 11 justices.
He was referring to an executive power known as "prerogative" which, under Britain's unwritten constitution, allows the government to take certain actions without going through parliament.
The government is appealing against a High Court ruling last month that it could not invoke Article 50 of the Lisbon Treaty, which begins a two-year exit process from the European Union, without parliamentary assent.
Prime Minister Theresa May has said she intends to trigger Article 50 by the end of March and the EU's chief negotiator said on Tuesday that would give a target of October 2018 for the Brexit deal to be agreed.
Eadie said by giving approval for June's referendum on whether Britain should leave the EU, parliament had accepted that Article 50 would have to be triggered in the event of a "leave" vote as that was the only way to implement the result.
Britons voted for Brexit by 52 to 48 per cent and the government has said this mandated it to begin the divorce process using the prerogative power.
