The UK Supreme Court has ruled that the government cannot trigger the Brexit process without consulting MPs, the BBC reports.
In December, the UK government appealed against a High Court ruling that Parliament must approve the Article 50 process of leaving the EU.
The Welsh Government told the hearing that if the sovereignty of Parliament was not upheld it would undermine the basis for devolution.
The UK government dismissed that argument as “tortuous”.
It claimed the prime minister could invoke Article 50 of the Lisbon Treaty to launch the formal two-year process of leaving the European Union using crown prerogative – powers used by ministers that do not require the permission of MPs.
But Wales’ top legal advisor, Counsel General Mick Antoniw, argued that starting this process would “modify the competence of the National Assembly for Wales and the Welsh Government”.
He said on Tuesday that if judges say legislation is needed a vote would be needed in the assembly.