UK Prime Minister Theresa May can only start Britain's divorce proceedings from the EU with Parliament's approval. Scotland's parliament and the Welsh and Northern Ireland assemblies, however, do not have a say.
By a vote of 8 to 3, the UK Supreme Court ruled on Tuesday that Prime Minister Theresa May's government cannot trigger Article 50 of the Lisbon Treaty without an Act of Parliament authorizing it.
The outcome of the Supreme Court case, heard by all 11 justices over four days in December, does not overturn June's referendum result, which saw 52 percent of voters favor leaving the EU.
Parliament will now be expected to vote on the bill within the next nine weeks in order to abide by the government's deadline for triggering Article 50, scheduled for no later than March 31. The consequent negotiations are expected to last up to two years.
What would happen legally if MPs vote against the bill remains unclear, as much of the constitutional law related to Brexit remains untested as yet.
No influence from devolved assemblies
In the hearing, Court President Lord David Neuberger said the British government had no obligation to consult the devolved assemblies in Scotland, Northern Ireland and Wales before beginning Brexit negotiations.
"UK ministers are not legally compelled to consult the devolved legislatures," Neuberger said, removing what could have been a potentially major obstacle to the UK's leaving the EU.
First Minister Nicola Sturgeon said the Scottish government was "disappointed" at the Supreme Court's ruling regarding devolved administrations.
"It is becoming clearer by the day that Scotland's voice is simply not being heard or listened to within the UK," Sturgeon said, adding that the ruling raised "fundamental issues" regarding Scotland's future as part of the UK.
The case against the UK government - brought by investment manager Gina Miller and hairdresser Deir Tozetti Dos Santos - argued that denying Parliament a vote was undemocratic.
"Today's decision has created legal certainty on our democratic process," Miller said, telling reporters that her victory was "not about politics, but process."
Attorney General Jeremy Wright said the UK government was "disappointed" by Tuesday's ruling but that it would "comply" and do "all that is necessary" to implement the judgement.
Tuesday's ruling raises hopes among pro-EU politicians that they will be able soften the terms of the UK's withdrawal from the 28-member bloc.
Some six months since the EU referendum, May finally outlined her intentions for Britain's future last Tuesday with a 12-point plan.
Addressing an audience of journalists and foreign ambassadors, May said the UK would not seek a deal that keeps Britain "half in, half out" of the EU, but a complete severance, which has been repeatedly referred to as a "hard Brexit."
Membership of the single market has been one of the key grounds for debate thus far, with many prominent remainers expressing concern about the economic ramifications of leaving. However, immigration - a key point for Brexiteers - cannot be reduced without exiting it.
Prime Minister Theresa May has also pledged that she will submit the final Brexit deal to both the House of Lords and the House of Commons.