Boris Johnson has said it would be “utterly pathetic” to cancel Brexit as he declared “people have voted for it, Parliament’s voted for it, on March 29 it’s going to happen”.

The former foreign secretary wrote off any chance of Theresa May’s Brexit deal coming back while assuring Brits that whatever terms are agreed with the EU “there will still be Mars bars”, “we will still have drinking water” and “the planes will fly”.

In a major speech which some may consider a veiled leadership move, Mr Johnson told his audience that rescinding Article 50 would be “so utterly pathetic” that it would “not only cause widespread international dismay but would also reinforce people’s view that there’s some kind of plot going on at Westminster to stop this thing”.

Speaking at JCB headquarters in Staffordshire, Mr Johnson also said it would be “shameful” for the Government to request an extension to the Brexit process beyond March 29.

Boris Johnson said it would be “utterly shameful” if Britain was to stay in the EU (EPA)

“It would be shameful at this late stage to change that totemic date – March 29, the one fact to which the public has been able to cling with absolute certainty in this sea of political confusion,” he said.

“All this vacuous talk of extending Article 50 is dishonest but it’s also weakening our negotiating position once again.”

He said he was confident that the EU27 would become more “flexible” in negotiations as the deadline for Brexit got nearer as history showed it was “only in the last few days and weeks of a negotiation that the big concessions are made”.

Talking of Mrs May’s deal, which was overwhelmingly voted down in Parliament earlier this week, Mr Johnson said: “It cannot come back. It is an ex-deal.”

“Under these proposals we would still be run by the EU… I think what we should be going for is the kind of clean, global Brexit that I have advocated,” he added.

He said the Government had not shown “sufficient drive or JCB-style gumption” in negotiations, adding that Mrs May should go back to Brussels and tell the EU that the UK would not accept the backstop arrangement.

Also on Friday Mr Johnson, who was the figurehead of Vote Leave in the 2016 referendum, attempted to distance himself from the campaign’s claims about immigration from Turkey.

Vote Leave produced adverts posted widely on social media during the campaign which stated that “Turkey (population 76 million) is joining the EU” and “Britain’s new border is with Syria and Iraq”.

Some of the ads included images of people in Turkey with large red arrows pointing towards the UK.

Challenged over the campaign ads, he said: “I didn’t say anything about Turkey in the referendum… Since I made no remarks, I can’t disown them.”

Mr Johnson added: “I’m not a nationalist if by that you mean I’m a xenophobe or someone who deprecates other countries and cultures – absolutely not, far from it, I’m called Boris.”

“My ancestors come from all over the place, like I don’t know – a JCB in fact, sourced I believe from around the world.

“In fact I’m probably even more cosmopolitan than a JCB.”