One of Europe's leading legal experts has warned that a transition deal between the UK and the EU needs to be agreed to avoid an economic "catastrophe."

Jean-Claude Piris, who was the head of the EU Council’s legal service between the late 80's and 2010, said that the complicated nature of a post-Brexit trade deal for the UK means that it would be impossible to tie-up a deal by the time the UK exits in 2019.

He told The Financial Times that "thousands of pages and hundreds of articles" need to be written as an agreement is drawn-up.

Despite UK PM Theresa May's comments that she expects a deal to be reached within two years - the man who was one of the architects of the Maastricht and Lisbon treaties says that it could take up to 10 years.

He added that this is "not the most pessimistic view because the most pessimistic view is that there will be no agreement at all.

"It will take years, that’s for sure. I don’t know how many years  [...] Some people say between four and eight years but that is if people have goodwill on both sides."

Failure to stike a deal before the UK goes means that general World Trade Organisation rules will apply - and that this would be damaging to both sides, with the UK feeling most of the pain.

"There will be the customs tariffs, there will be controls at the borders and so forth [...] The UK will also lose the benefits of trade agreements made by the EU with 60 countries in the world. You’ll lose also the financial passport."

Mr Piris says the UK could strike a deal to stay in the Single Market during the transition period - but this would come at the cost of potentially politically-difficult concession on the movement of people. Another compromise could be a deal to stay in the customs union (this is the current status of Turkey) which would protect a number of its standing trade deals - but this would give the UK less freedom to create its own bi-lateral deals.