Today’s Supreme Court judgment means that the government has to ask Parliament to pass a law in order to begin the process of leaving the EU. This process begins by triggering Article 50 of the Treaty on European Union.
What the government argued
The government argued it has always had the power to make and leave treaties with other countries.
It said that it should be able to do the same with the treaties keeping us in the EU.
What the Supreme Court said
A majority of Supreme Court disagreed. Eight of the 11 judges decided that the EU treaties are different for two reasons.
First, parliament passed a law in 1972 so that we could join the EU. Leaving would stop that law from working.
What about the referendum?
Last June a majority of people voted to leave the EU.
The Supreme Court said that it was an important political event.
But the law that set up the referendum didn’t say how we would actually leave. That’s why this court case happened.
The judges decided that the only way that the UK can make the referendum result a formal reality is by parliament passing a law.
The government can’t carry out the result of the referendum on its own.
What about Scotland?
The Scottish and Welsh governments, as well as campaigners from Northern Ireland, argued that they should have a say in triggering Article 50 as well.
They said that leaving the EU affects the powers of governments in those parts of the UK. There’s a political rule that the Westminster parliament won’t make laws affecting Scotland, Wales and Northern Ireland without getting their permission.
But the court said that this is only a political rule, not a legal one. So Scotland, Wales and Northern Ireland don’t have a veto over Brexit.
The government has now said it will put a bill triggering Article 50 to parliament "within days". This gives MPs and Lords a chance to try to change the bill and control how Brexit plays out.
The government will want the law passed quickly if it’s going to start the process of leaving the EU by the end of March.