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An important decision was announced on Tuesday morning about the way that the UK can officially start the process to leave the European Union (EU).

The Supreme Court, which is the highest court in the country, has announced that UK Prime Minister Theresa May will have to ask for Parliament's permission before she can officially start the process for the UK to leave the EU.

All 11 Supreme Court judges came together to decide this, which has never happened since the group was created in 1876 (although it was called something different back then). This shows how significant it is.

Theresa May argued that she had the power to start the process herself, without asking Parliament's permission.

But others thought that she should have to ask Parliament first - and the Supreme Court has agreed with them.

The Supreme Court's decision is final.

It also ruled that the UK government - led by Theresa May - does not need to ask governments in Scotland, Wales and Northern Ireland before starting the process.

While UK adults have voted that they want to leave the EU, the official process to leave hasn't started yet.

Find out more about each side of the argument below.

Ever since the UK voted to leave the EU, there have been discussions about how the UK could officially start the process to leave the group.

Theresa May wants to start the process by doing something called triggering Article 50. Ricky explains what this is below.

She didn't think she needed to ask the rest of Parliament for permission to do this because the UK government has something called 'prerogative powers'.

This is an old type of power from when kings and queens could basically do what they wanted.

But not everyone agreed that this was the case.

Others thought Theresa May should have to ask for Parliament's permission before she can trigger Article 50 and officially start the process for the UK to leave the EU.

This is because of how much it could affect the lives of people who live in the UK.

The case also involved the governments in Scotland, Northern Ireland and Wales.

They said they had a right to be involved in certain Brexit decisions because of how the result could affect people living in their countries, which make up the UK.

Not everyone agreed on how much say these other governments should have.

On 3 November, the High Court - which is a court below the Supreme Court - decided the government did not have the power to trigger Article 50 without getting approval from Parliament.

But the UK government did not accept this decision.

So, the Article 50 issue had to then be decided by the most important court in the country, the Supreme Court.

They agreed with the High Court - and have said that Theresa May will have to ask for Parliament's approval before she can trigger Article 50.

Their decision is final.

Parliament will now get to have a say before Theresa May can trigger Article 50.

It is expected that she will hurry to get any law needed to do this sorted.

Then she can have the vote in Parliament and hopefully trigger Article 50 before the end of March, as this is when she has said she wants to do it.

When Parliament votes on triggering Article 50, it is not thought that they will block it.

Jeremy Corbyn, who leads the main party against Theresa May (the Labour party), has said he will be telling his party to vote to allow the government to trigger Article 50.

There will be some people - for example, politicians from the Scottish National Party - who will vote against it, but it is not thought there will be enough of them to stop it from happening.

While Scottish First Minister Nicola Sturgeon "welcomed" the decision that Theresa May will need to seek approval from Parliament before triggering Article 50, she said she was "obviously disappointed" it was decided that the UK government does not need to ask the Scottish parliament about it.

Once Article 50 is triggered, the UK will not be able to come back into the EU unless all the other 27 countries in the group agree that it can.