EU referendum: Boris Johnson calls the EU 'a jail with the door left open' and says it would be 'wonderful' to leave

London Mayor tells Andrew Marr it would be 'wonderful' to leave the economic bloc - but struggles on specifics

In this handout image provided by the BBC, London Mayor Boris Johnson appears on The Andrew Marr Show on March 6, 2016 (Getty Images)

Boris Johnson has said being in the EU is like being in a jail where “the jailor has accidentally left the door open”, telling the BBC’s Andrew Marr Show that leaving would be “wonderful”.

The London Mayor put the case for Brexit in effusive terms, calling the referendum scheduled for the 23 June a “once in a lifetime opportunity that will not come again”.

Mr Johnson said he had experienced first-hand how EU directives impacted Britain, saying it took UK officials a year to convince Brussels that the Crossrail project didn’t need 50 per cent wider tunnels to accommodate German trains.

And hitting out at what he described as the negative tactics of the Remain campaign, he said: “This is like the jailer has accidentally left the door of the jail open and people can see the sunlit land beyond.

“And everybody is suddenly wrangling about the terrors of the world outside. Actually it would be wonderful. It would be a huge weight lifted from British business.”

Mr Johnson spoke at length about Britain’s “proud democracy” and the need to “take back control over our borders and control over our democracy”.

But in an at-times acrimonious interview, the Tory MP struggled to focus in on the specifics of what leaving the EU would mean for the UK.

Mr Marr asked repeatedly if Mr Johnson was still in favour of the idea of the single market – as he has said before – and whether he wanted Britain to still be part of it outside the EU.

And Mr Johnson also refused to be pinned down on exactly who he was accusing when he said business leader John Longworth had paid a “heavy price” for expressing his support for Brexit.

Asked about the way he came out in favour of leaving the EU, Mr Johnson said he was involved a process of seeking a new wording of the clause that would return sovereignty to the UK.

He said that the Government’s lawyers “exploded” the entire document, saying it could never work with EU treaties – and confirmed that was the moment he decided to support Out.

That may well anger David Cameron and the Cabinet, who were led to believe Mr Johnson made a last-minute decision on Sunday evening two weeks ago after hearing the Prime Minister speak himself on the Andrew Marr Show. Downing Street has previously criticised the way the Mayor conducted the announcement.

Meanwhile, asked if he expected to become the next Prime Minister, Mr Johnson briefly obfuscated before giving his clearest answer of the entire morning.

“Certainly not,” he said. “We have a wonderful Prime Minister, and to the best of my knowledge there is not a single European leader who has had to step down because of a referendum, on Europe or not.

"The whole thing is cobblers, you are just trying to personalise it to make it more interesting for viewers,” he added.