The former director general of the World Trade Organisation has mocked Britain’s Brexit chiefs, claiming that Boris Johnson was a “nasty young kid” and that “all ministers should read their briefs” in response to David Davis’s failure to examine the impact of crashing out of the EU.

Pascal Lamy, who was the chief of WTO between 2005 and 2013, also said that a comprehensive free-trade deal between the UK and the EU could take between five and six years to negotiate, casting doubt on the two-year timeframe permitted under Article 50.

“Many of us, including me, believe that there is no way of removing the egg from the omelette can be done in two years,” the former EU official said at an event in London.

But when asked whether personalities on either side of the Channel would make a difference to the Brexit negotiations, Mr Lamy replied: “Let me be frank, what I will say to you from knowing Boris as a child… because Stanley Johnson his father was living… they were near to where we used lived. So I saw Boris as a nasty young kid.”

“He never changed,” he added, before saying he believed the eccentricity of the British would not be a problem in the complex negotiations. “We credit the British with a very specific eccentricity. Imagine someone from Sweden or Norway acting like Boris.”

A friend of Mr Johnson's told The Independent: "I don't think Boris will lose any sleep over this as he has a important job to do and is giving it 100 per cent to get the best deal for the UK.

"It's a shame that Mr Lamy can't talk about substance and the positive things that the Government wants to achieve from Brexit like Boris and the Government are doing.

"I am afraid his personal attack just shows he can't accept Britain has a strong voice and used it last June."

Mr Lamy’s comments – during an event hosted by think-tank the Institute for Government – also came after David Davis, the Brexit Secretary, admitted during a select committee hearing on Wednesday that the Government had done no economic assessment of negotiations with EU leaders breaking down.

When asked by The Independent whether this was irresponsible, Mr Lamy, who was also chief of staff to Jacques Delors during his tenure as President of the European Commission, replied: “All ministers should read their briefs. I’m absolutely certain knowing the politics of the UK civil service that they have provided their ministers with scenarios of this.

“I’m sure the job has been done and there is an evaluation on the UK side on what could happen.”

On the issue of the Britain’s so-called divorce settlement – likely to be a contentious issue throughout the “costly and complex” negotiations – Mr Lamy added that there could be the possibility of a “compromise”. He estimated this could be between €40bn (£34bn) and 60bn.

Speaking as Theresa May ruled out a second Scottish independence referendum by saying “now is not the time” for another vote, Mr Lamy also insisted the country would have “zero technical problems” rejoining the EU after Brexit.