Brexit: A long, complicated divorce

A British exit from the European Union would plunge the two parties into a messy divorce and force them build a new relationship after a marriage of more than 40 years.

Here are the logistical and legal details of how the split would work, and the key issues that Brussels and London would have to negotiate if Britons vote for a "Brexit" in the June 23 referendum.

- The legal basis -

The procedure for a country to leave the EU is built into the bloc's treaties.

Article 50 of the 2009 Lisbon Treaty says that "any member state may decide to withdraw from the Union in accordance with its own constitutional requirements".

In the event of a "Leave" vote next month, British Prime Minister David Cameron has said the government would "straight away" trigger Article 50, beginning the negotiation process to leave.

- How long would it take? -

There is a two-year time period to negotiate the arrangements for exit under Article 50, with a "Brexit" then happening automatically if there is no deal for the new form of relationship.

But in practice it could take a lot longer. Britain and the EU can agree to delay the exit until a better deal has been reached, although a delay must be approved unanimously.

European Commission President Jean-Claude Juncker has said Britain would be "a third party, who we won't be bending over backwards for".

The final deal would then have to be approved by a qualified majority of the Council of the European Union, which groups the member states, after the approval of the European Parliament.

- A decade of uncertainty? -

The British government warned in a report in February that it could take up to 10 years to tie up all the loose ends.

"It is... probable that it would take an extended period to negotiate first our exit from the EU, secondly our future arrangements with the EU, and thirdly our trade deals with countries outside of the EU, on any terms that would be acceptable to the UK," it said.

"In short, a vote to leave the EU would be the start, not the end, of a process. It could lead to up to a decade or more of uncertainty."

- Norwegian or Swiss? -

The simplest and most frequently cited option is for Britain to join Iceland and Norway as members of the European Economic Area, which would give them access to the single market. 

That however would mean London still having to obey the EU's rules despite no longer having any say in how they are formed, plus still having to pay money to Brussels.

The Swiss model is another popular one for pro-Brexit campaigners but it is "not likely that Britain would take this route", said Jean-Claude Piris, a former senior EU jurist who is now a consultant.

Piris pointed out that Switzerland has had to sign hundreds of trade agreements with the EU and that Brussels is still not satisfied with the relationship.

Other options include a free trade deal with the EU or a customs union similar to that between Turkey and the EU. Failing that it would simply become a trading partner like the United States or China.

Whatever happened, Britain would end up a "sort of satellite to the EU", said Piris.

- Britons abroad -

London would have to negotiate the status of the two million Britons living or working in the EU.

This would particularly affect their pensions and rights to healthcare, with the British government noting: "UK citizens resident abroad, among them those who have retired to Spain, would not be able to assume that these rights will be guaranteed."