Boris Johnson, Vote Leave campaigner
Top Conservative Boris Johnson launched Vote Leave's battle bus tour in Cornwall on 11 May Getty

The EU referendum campaign has taken a nasty turn as 'remain' and 'leave' campaigners hurl insults at each other over the 23 June ballot. Brexit backer Boris Johnson is the latest politician to make a jibe at the opposition, branding David Cameron "demented" for suggesting a 'leave' vote could trigger a conflict on the continent.

"Only a few months ago the prime minister was saying that people were needlessly scaremongering about leaving the EU and there's no doubt that we could do a free trade deal," the former Mayor of London told Sky News.

"That was in the days when he was saying actually he might consider leaving himself. Obviously things have changed a bit since then. I think all this talk of World War Three and bubonic plague is totally demented, frankly."

The comments came as the Conservative MP launched the Vote Leave bus tour, which will see the vehicle travel across the UK ahead of the referendum.

Johnson first visited Cornwall, where he was photographed brandishing a pasty. His attack against Cameron comes a day after Alan Johnson accused Brexit campaigners of being "extremists".

"They cannot find anything good to say and I think that's extreme," the Labour in For Britain chief told journalists. "We can all find things that are wrong with the EU, but they cannot find anything that's right – and that suggests a certain mentality that is not rational and is not balanced.

"The majority of British people have a rational balance, [and a] moderate approach to this question."

Elsewhere, 'remain' advocate Gordon Brown made his first speech of the EU referendum campaign on 11 May. The former Labour prime minister said he was "happy" to go head-to-head with Boris Johnson in a debate.

"I'm happy to take anybody on these days," he told reporters. "I was part of the Scottish referendum campaign, and I'm not so sure that Boris was on our side on that either."

EU referendum: David Cameron says UK will keep peace in Europe by avoiding BrexitIBTimes UK