Brexiteers’ dream of a second Elizabethan age of trade after Britain leaves the EU is nothing more than “buccaneering blather”, Britain’s former top EU diplomat Sir Ivan Rogers warned on Wednesday.

In a lengthy and excoriating critique of the entire debate around Brexit, the former top EU adviser to both David Cameron and Theresa May warned that the entire debate around Britain’s future was based on “extraordinary misconceptions” of what was actually available.

He also accused Mrs May of an "act of folly" in setting red lines on the customs union and single market membership that ruled out a host of options for Britain.

Taking aim at the fallacies pedaled by both Brexiteers and Remainers, Sir Ivan warned that the government was in danger of “miss-selling the British people” over the real Brexit options they face.

He called for a debate based on a clear-eyed evaluation of pros and cons, “not on fantasies, or incoherent and muddled thinking”.

“No trade policy with third countries, however successfully aggressive, will deliver very quick results, or ones, which on any serious analysis will transform the UK’s productivity performance and economic prospects.

“More than 65 per cent of all UK exports are, after all, to the EU, or to countries with whom we already have a preferential deal via EU membership,” he said.