THERESA May's Brexit blueprint is a "pipe dream" and there has been "culpable naivety" in the negotiations, the UK's former ambassador to the European Union has said.

Sir Ivan Rogers said neither the Prime Minister's Chequers plan for future relations with the bloc or Boris Johnson's alternative had any chance of being agreed by Brussels.

In a speech in Cambridge, the ex-diplomat, who quit last year, said there had been no plan for Brexit but plenty of "plausible bullshit" instead.

The National: Sir Ivan Rogers has quit the civil service with immediate effect (AP)

Rogers (pictured above) said there had been "culpable naivety" on an almost daily basis over the last two years as the UK makes demands that Brussels will not accept.

He dismissed the PM's Chequers plan for Brexit and the Canada-style deal being floated by former foreign secretary Mr Johnson, saying the chance of the EU agreeing to either was "precisely zero".

He said: "Nearly two and a half years on from the referendum, we are, both on the EU deal, and on other post Brexit trade deals, still lost in campaign mode on fantasy island.

"But the time for these fantasies is long past."

He added: "We needed seriously to work through precisely how a Brexit process could be made to work before launching it."

Rogers branded Johnson's so-called Canada plus plus proposals as "fiction" because they are "not the pluses anyone else will agree".

"The Johnsonian Canada plus plus is as big a pipe dream as Chequers," he said.

"In some respects, rather bigger."