A European Court of Justice (ECJ) advocate general has issued a legal opinion that the UK can unilaterally revoke its withdrawal from the EU.

Here's everything you need to know following the historic decision

- What will happen after the opinion was issued on Tuesday?

Advoate General Manuel Campos Sanchez-Bordona said Article 50 allows the "unilateral revocation of the notification of the intention to withdraw from the EU, until such time as the Withdrawal Agreement is formally concluded". His opinion is not binding on the court. The Judges of the Court are now beginning their deliberations in this case and will issue a judgment at a later date.

READ MORE: ECJ Advocate General says UK can revoke Article 50 unilaterally 

- Who is involved in this legal challenge?

The case was initially brought to court by a group of politicians from four parties, working in three different parliaments. The current group features Green MSPs Andy Wightman and Ross Greer, SNP MEP Alyn Smith and Labour MEPs David Martin and Catherine Stihler.

The politicians are supported by campaigning lawyer Jolyon Maugham QC, director of the Good Law Project. The case, formally known as Wightman and others v the Secretary of State for Exiting the European Union, is opposed by the UK Government.

- What do the cross-party politicians want?

In March 2017, Prime Minister Theresa May started the formal process for the UK to leave the European Union (EU) when she triggered Article 50 of the Treaty on European Union - the legal mechanism for a country to leave the EU.

The politicians want judges at the European Court of Justice (ECJ) to indicate whether the UK can unilaterally revoke that Article 50 request to leave the EU. In other words, can the UK stop the Brexit process on its own, without the consent of the other 27 EU member states?

READ MORE: "We have a roadmap out of the Brexit shambles" - says cross-party group post ECJ ruling 

As the Good Law Project puts it: "Many experts believe we can choose to stay in the EU without permission. That, if we want, we can just withdraw our notice. But there is only one way to be sure: a court has to decide what Article 50 means." Mr Maugham argues the case "could decide the fate of the nation".

- How can they afford it?

The campaigners launched a crowdfunding drive, which is close to reaching its £200,000 target after receiving donations from more than 5,800 people. Their legal team is said to be working at a "heavily discounted rate".

HeraldScotland:

- What does the UK Government say?

It has maintained the question being asked is entirely hypothetical and academic, and challenged the cross-party group at every legal stage. The UK Government insists it is committed to implementing the result of the EU referendum in June 2016 and has no intention of revoking Article 50. Ministers have argued it is not for courts to enter into a hypothetical or premature debate.

- How did the case get all the way to Europe?

A ruling from European judges is what the group has sought from day one. The politicians have overcome a series of legal hurdles at every stage in the process, which first came to court in February. At the Court of Session in Edinburgh, they lost an early bid to have a full hearing on the matter, and later secured one on appeal.

READ MORE: Joanna Cherry: Why we must know if MPs have the power to revoke Article 50 

Following that full hearing, the court knocked back their bid to have the issue referred to the ECJ. Again, this was overturned by senior judges in Edinburgh upon appeal and the case was allowed to proceed to the ECJ. A further attempt by the UK Government to stop the case getting to Europe was rejected earlier this month at the UK Supreme Court.

- What did the ECJ say? 

A statement from the ECJ said: "In answer to the question from the Scottish court, the Advocate General proposes that the Court of Justice should, in its future judgment, declare that Article 50 TEU allows the unilateral revocation of the notification of the intention to withdraw from the EU, until such time as the Withdrawal Agreement is formally concluded, provided that the revocation has been decided upon in accordance with the member state's constitutional requirements, is formally notified to the European Council and does not involve an abusive practice." 

- What does that mean for Parliament and can it stop Brexit?

The campaigners say it means the UK Parliament will get clear guidance from the European Court of Justice about the precise powers open to it as it votes on the deal. Scotland's most senior judge made the different roles of the parliament and court clear when he ruled in September that the case could go to Europe.

He said the ECJ would not be advising Parliament on "what it must or ought to do". Instead, he said it would be "merely declaring the law as part of its central function". How Parliament chooses to react to that judgment is entirely a matter for it. 

Legal representatives for the UK Government believe the case is inadmissible as it deals with a hypothetical situation, as the UK Government's policy is not to revoke Article 50.

- What did those involved say?

SNP MEP Alyn Smith, one of those who brought the case, said: "This is a huge win for us, and a huge step forward from the highest court in the business, and confirms what we have been hoping for: that the UK can indeed change its mind on Brexit and revoke Article 50, unilaterally. 

"The Advocate General Opinion is not the final judgment, but the practice of the ECJ is that the judges tend to follow the Opinion, so this is a major landmark.

"We now have a roadmap out of the Brexit shambles, a bright light has switched on above an 'Exit' sign and the false choice being offered to MPs at Westminster - that it is Mrs May's disastrous deal or chaos - is shown for what it is, an abuse of Parliament."

Advocate General for Scotland Lord Keen QC, representing the UK Government, said the case is a "hypothetical validity challenge" and those behind it seek "political ammunition to be used in and to pressure the UK Parliament".