The Counsel General for Wales is asking the Supreme Court to hear the Welsh Government's arguments against the UK Government's wish to trigger the process of leaving the European Union without parliamentary approval.
The Welsh Government wants the UK Government to put any Brexit plans to Parliament and also consult the Welsh Assembly, the Scottish Parliament and the Northern Ireland Assembly.
But Counsel General Mick Antoniw argues that much more is at stake. UK Ministers want to trigger Article 50 of the Treaty of Rome and so begin the process of EU withdrawal under the Royal Prerogative, despite the Act of Parliament that took Britain into the European Union in the first place.
Mr Antoniw argues that such a use of the Royal Prerogative threatens the Welsh Government and Assembly, which were established by the Government of Wales Act. He says the questions before the Supreme Court "raise issues of profound importance":