LONDON (Reuters) - What happens if the British government loses its appeal against a High Court ruling that said ministers could not start the process of leaving the European Union without parliament's approval?
All 11 Supreme Court judges sat on the appeal, the first time they have done so since its formation in 2009, and its president said on Thursday it would reach a decision as quickly as possible.
One suggestion is that both houses of parliament would debate and vote on a short motion giving Prime Minister Theresa May authority to trigger Article 50, kick-starting the process.
However, lawyers leading the challenge against the government argue that only a new law authorising the government to trigger Article 50 would suffice because citizens would lose rights granted by the parliamentary act by which Britain joined the EU.
Only a new law could start a process which would inevitably take those rights away, the lawyers say.
Even the government's own lawyer told the court that a one-line bill would probably be necessary. If that is the case, it is not clear how much opposition or delay such a bill would face.
Most MPs in the lower house of parliament will now back Brexit, and they overwhelmingly supported May's timetable for leaving the union in a non-binding vote on Wednesday.
Some might try to force amendments and conditions on May while the upper unelected House of Lords, many of whose members opposed Brexit, could put up opposition.
Judges cannot order parliament to pass laws, so they themselves might avoid suggesting a law is required.
"So the real question is, can we as a court say to parliament, the trigger stage, a motion would not be good enough, even a motion supported -- a unanimous motion, that would not be good enough," Robert Carnwath, one of the Supreme Court judges, said during the appeal.
"There has to be this one-line bill that says: yes, you can trigger."
If the court backs the government appeal, then May will be able to go ahead with her plan to trigger Article 50 by the end of March without seeking specific backing from MPs.
Investors believe that greater parliamentary involvement would reduce the chances of a "hard Brexit" in which tight controls on immigration are prioritised over European single market access.
Reporting by Michael Holden; editing by John Stonestreet