Prime Minister Theresa May's government is set to launch a challenge against a court ruling that it requires parliamentary approval to start the process of leaving the European Union, a decision that could upset Britain's Brexit plans.
If the Supreme Court, the United Kingdom's highest judicial body, dismisses the government appeal which begins on Monday, it could derail May's timetable for triggering Article 50 of the Lisbon Treaty and leaving the EU.
The government's legal fight comes against a backdrop of claims by some politicians and newspapers that establishment judges want to thwart the Brexit process.
It will be the most high-profile and complex case the court has considered since it came into being seven years ago and is due to last for four days. For the first time all its 11 justices will sit on the panel with the verdict due later in January.
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If May wins, she can proceed with her plans to invoke Article 50 by the end of March.
But if she loses, parliament could in theory block Brexit as most lawmakers (MPs) supported staying in the EU in a referendum in June, though few observers expect such an outcome. Even so, lawmaker approval could open the process to greater scrutiny and delay.
Investors believe the greater parliament's involvement the less chance there is of a "hard Brexit" in which tight controls on immigration are prioritised over European single market access.
