MPs are set to vote on up to four amendments to the Prime Minister's proposed delay to Brexit by extending Article 50 on Thursday afternoon.

Speaker John Bercow called four amendments, including one calling for a second referendum, to be debated in the Commons.

Read more: As it happens: MPs vote on Article 50 delay

Some of the amendments call for an extension to Article 50.

Article 50 of the Treaty of Lisbon allows any EU member state to leave the European Union, and outlines the procedures they would have to follow to do so.

Negotiations are supposed to only last two years, so what will an extension mean for the UK and the EU?

Here's everything you need to know about the parameters of extending Article 50:

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What types of extension are possible?

The motion states that there are two potential types of extension.

The first is a short, technical extension: this means that if a deal is approved before the March European Council, domestic legislation will be allowed to pass in order to implement a deal.

Read more: MPs reject no-deal Brexit: What happens next?

The second longer extension will come into effect if a deal is not approved before the March European Council. This will then allow time for the Government and Parliament to determine a course of action for the UK to follow. This extension would therefore require the UK to participate in the European Parliament elections.

What are the legal requirements for extending Article 50?

The Article 50 period is currently set at two years, but this can change if "the European Council, in agreement with the Member State concerned, unanimously decides to extend [it]".

The article itself does not establish an upper limit on the length of an extension, however considering the period at hand is time-limited, any extension would have to set a specific end date.

This is because it is necessary for 'reasons of legal certainty' to be clear on the date that the UK will leave the EU.

Read more: Theresa May offers MPs Article 50 delay vote

During an extension, the UK will continue to hold the status of a full Member State, subject to all rights and obligations set out in the EU Treaties and under EU law.

This also means that the UK could not use this extended period to continue negotiations on its future relationship with the EU; these terms can only be negotiated after the UK has officially left.

What conditions are in place from the EU?

The EU have only established one condition on the extension; they expect a 'credible justification' for an extension and its duration. This was outlined in a statement issued on behalf of the President of the European Council on Tuesday.

EU leaders, including Mark Rutte and Emmanuel Macron, as well as Michel Barnier and numerous Members of the European Parliament, have also set out this requirement.

How would the UK request an extension?

Article 50 does not actually outline how either party would request for an extension.

However, the current government believes it would be appropriate for Theresa May to personally write to the President of the European Council, with a clear indication on the reasons behind the extension and how long it would last. This would therefore meet the conditions given by the EU.

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How would the EU accept an extension?

In order to accept a request from the UK to extend Article 50, European Council would have to approve the decision by unamity. This means that the Heads of State or Government of the other 27 EU Member States would have to agree to the terms.

Again, Article 50 does not set out specifically how the decision should be taken, and the European Council could technically do this through a written procedure. However, up until this point, they have taken all decisions under Article 50 in scheduled meetings, and it's assumed they'll do the same with the extension.

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Therefore, the UK must request any extension before the March European Council, which akes place on March 21 and 22. It is the last scheduled meeting before the end of the current Article 50 period, and means appropriate debates and decisions can take place.

Would the UK have to take part in the European Parliament elections?

It's a main concern for many people, and the short answer is yes; if the UK were to request an extension beyond July 1, we would have to participate in the European Parliament elections.

Elections are set to take place across the EU between May 23 and 26, and if we were to remain a Member State after the new parliament meets for the first time on July 2, we would have to particpate for two reasons: 

  • First of all, the EU Treaties states that EU citizens have the right to be represented in the European Parliament, and the UK could not legally return MEPs to the new parliament without participating
  • Secondly, in order for the Parliament to appoint the Commision and adopt any legislation, it needs to be properly constituted under the EU Treaties, with duly elected representatives from all Member States

Can the UK implement a short extension initially, and extend it later?

Not really. If the UK were to take part in the European Parliament elections, a short extension would make any subsequent extension impossible.

If the UK were to avoid taking part in the elections, it would not have any duly elected MEPs when the new parliament plenary begins on July 2. Without UK MEPs, the European Parliament would be improperly constituted; this would therefore put the 'functioning of the EU's institutions at risk'.