On Wednesday European Commission president Jean-Claude Juncker told the European Parliament that if Britain leaves the EU, it would still be welcome to re-join under Article 49 of the Treaty of the European Union.

This comes a day after Mr Juncker and Donald Tusk, the European Council president, said that Britain would still be welcome to reverse Brexit now if it chose to do so.

What is Article 49?

In Britain, Article 50 – which allows a state to leave the EU – is the far more famous cousin of Article 49. Article 50 spells out the process by which a state leaves.

Article 49, by contrast, is the opposite: it spells out how countries may join the EU. In other words, it is the normal way for a country that isn’t in the EU to join the bloc, and spells out the conditions under which it might do so.

So can Britain trigger Article 49 to reverse Brexit?

Not unilaterally – once Britain has left, it is no longer a party to the Treaty of the European Union and cannot “invoke” the clause in the same way Article 50 was triggered.

Rather, the clause simply sets how existing members of the EU may admit a new member to join.

What are the conditions of Article 49?

First, a country wishing to join must submit its request to the European Council – the heads of state and government of the members of the EU. Currently there are 28, when Britain leaves there will be 27, though there could be more if other countries join after the UK leaves.

The Council must approve the application unanimously. It also requires the consent of the European Parliament – essentially a vote of all the MEPs, which are elected by member states.

Article 49 also states that the decision must be taken in consultation with the European Commission – Mr Juncker’s institution, which is essentially the EU’s combined bureaucracy and executive.

What if we want to stop Brexit before it happens?

This would be a case of revoking Article 50, rather than triggering Article 49. There are differing legal opinions on whether Article 50 can be unilaterally revoked by a member state, but Mr Juncker and Mr Tusk have said they would support Britain remaining if it decided to do so today, so this should not be a problem, as long as both sides agree.

Would anything be different if Britain rejoined the EU under Article 49?

On paper, the EU would be under no obligation to offer Britain its budget rebate, or its opt-outs from various areas of EU law, such as joining the euro – the UK would be treated as a new country joining the bloc.

However, nobody in the Commission has actually explicitly spelled out whether Britain would keep either of these things. It is likely that they would be up for negotiation.