Extricating Britain from the EU will be “as difficult as removing an egg from an omelette”, and will almost certainly take far longer than the two-year transition period due to begin in March 2019, the former director-general of the World Trade Organisation has warned.

Giving evidence to the Brexit committee in Parliament, Pascal Lamy, who led the WTO between 2005 and 2013, said that he was “now free enough to say what I think”.

He went on: “Trade-wise Brexit was like removing an egg from an omelette. The UK is in the internal market part of the omelette, which was stirred for many, many years, and moving back from that is inevitably a very complex issue technically, leaving aside the politics.

“Getting this done is as difficult as getting an egg out of an omelette. Which is why I think it will take a long time.”

Mr Lamy had previously said that the UK would probably not be able to reach a final trade deal with the EU, the destination of around half our exports, for possibly five or six years, and that Britain would probably have to ask to extend its two-year “implementation period” which ministers are currently negotiating with the EU.

“There is a serious risk that this is too short to come to a full-fledged new trade arrangement,” Mr Lamy told MPs.

“I would not be surprised if this transition period, two years, would be too short.”

What is the Brexit 'transition'?

Asked what would happen if the clock on the two-year transition ran down and there was no final trade deal, Mr Lamy – who was also an EU trade commissioner between 1999 and 2004 – said that the UK would have to choose between a cliff-edge Brexit, in which tariffs would go up, and requesting an extension.

“You would have to repeat what you just did, which is have a longer transition in order to avoid the famous ‘cliff’, which is a change which moves the UK into WTO terms, which is what both sides want to avoid,” he said.

A leaked document last week suggested UK officials want to keep open the possibility of extending the transition period – although this idea was attacked by hardline Brexiteer MPs.

“The UK believes the period’s duration should be determined simply by how long it will take to prepare and implement the new processes and systems, that will underpin the future partnership,” the note said.