We’ve had help answering this question from our friends at the Institute for Government.
The answer is yes to both of these questions.
Article 50 of the Treaty on European Union sets the procedure for leaving the EU. The UK triggered Article 50 (following parliamentary approval) in March 2017, which set off a two-year countdown before we officially leave, on 29 March 2019.
If the UK requested an extension of this process, the Article 50 deadline could be extended for a specified period of time, but only if each and every one of the 27 other EU member states agreed. The UK would also need to pass legislation to change the date of Brexit currently in UK law.
The UK’s notification to the EU under Article 50 could also be cancelled, meaning we would not leave the EU. The Court of Justice of the European Union ruled on 10 December that the UK can revoke its notification by itself, without requiring the agreement of other EU member states.
There are two main conditions on a country's right to revoke their Article 50 declaration. The first is that it is time limited: it can only happen so long as "a withdrawal agreement has not entered into force". If a withdrawal agreement is not reached between the country and the EU, it is limited to the two year window for withdrawal set out out in Article 50 itself.
MPs are scheduled to vote for the first time on whether or not to approve the draft withdrawal agreement on 11 December.
The second is that the country must have "taken the revocation decision in accordance with its constitutional requirements". In the case of the UK, this could mean that parliament would have to vote to authorise the government to revoke Article 50, just as they had to vote for the government to trigger Article 50 in the first place – although some reports suggest the government's legal advice is that they would not need parliament's approval.
This article is part of our Ask Full Fact series on Brexit, answering your questions about Brexit and the latest negotiations between the UK and the EU.
Update 10 December 2018
This article was updated in light of the 10 December ruling from the Court of Justice of the European Union.
With Brexit fast approaching, reliable information is crucial.
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