Triggering Article 50 does not mean it cannot be later revoked, according to a leading EU lawyer who helped write it, who also warned of “15 years of economic pain” immediately after the UK leaves the European Union.

“My opinion is that there is no legal provision in Article 50 providing that when you give your intention you cannot change your intention, so I think it’s possible legally,” lawyer Jean Claude Piris told Sky News.

Mr Piris was a key figure in the drafting of both the Maastricht and Lisbon Treaties, the two documents that formally created European Union. Until Lisbon, which was signed in 2007, no formal process existed by which a country could exit the EU.

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“Nobody thought that it would be used,” he said. “But people thought that if it would be used one day it would probably be by the United Kingdom, because as you know the United Kingdom has always been a little bit in, little bit out.

“Difficult things will begin after the exit, when you go out, you have no trade agreement whatsoever, with nobody, because you are losing the fact that you were a member of the single market, and you were participating to the 60 or 70 agreements with a lot of countries in the world.

“You know a trade agreement is very long. It is very heavy, thousands of pages.

“If you don’t want a European Court of Justice looking at what you are doing. If you don’t want somebody telling you that you’re obliged to have immigration from EU, if you want to be able to negotiate trade agreements alone and not with the others, then okay, fine, but there will be a price, and the price will be certainly heavy. It might be good in thirty years, but for the fifteen years to come, definitely not.”