Launching Labour's pro-EU battle bus tour, Mr Johnson said the "extremist" tag was not incendiary but accurate, because those who wanted Britain to withdraw believed everything about Brussels was bad.

"It's an extreme view that there is absolutely nothing good about the EU at all. "It's extreme - not to take the view that we ought to leave - but the view that you cannot find anything good to say about an institution that has done many good things, won the Nobel Peace Prize in 2012, is admired in terms of its role in keeping the peace on our continent, you can find nothing good to say about it whatsoever. "It is like if I was to come in here and say we ought to leave Nato, and here's all the reasons, and Nato has done nothing good, I would be classified as an extremist. "We can all find things that are wrong with the European Union, but they can't find anything that's right - and that suggests a kind of, a certain mentality, that is not rational, and not balanced."

Mr Duncan Smith hit back by accusing the Labour former cabinet member of engaging in "threats" and being "ridiculous".

Key points from Iain Duncan Smith's speech

EU has become 'a friend of the haves rather than have-nots'

Cameron failed to deliver a reformed EU

Britain is powerless to prevent further migration

Immigration putting pressure on housing, schools, NHS

Big corporates in Germany benefit, small firms in Britain lose out

Voting to leave will be shock the EU 'desperately' needs

Mr Duncan Smith said:

"I don't know in what world it is extreme to want to have your democracy back, power over what you do, control over your laws and the power to make decisions about your people, elected by British people and rejected by British people when you get it wrong. "If someone wants to tell me that's an extreme position, then I want to know what defines that. "It is not extreme to want democratic government in your country, to be responsible to the electorate and to make the laws that best help them rather than have 60% of those laws made in Brussels. "Those people in Remain really need to stop throwing threats and ridiculous terms like that around. It just demeans them, and it demeans the debate."

Labour leader Jeremy Corbyn also used the launch to insist he was fully behind the Remain campaign despite his own "many criticisms" of the EU.

"I've made many, many criticisms of the European Union; I still make those criticisms," Mr Corbyn said as he insisted he backed the Remain stance in order to defend workers' rights and the environment.

"We see it as an act of solidarity with people who think like us across Europe; going it alone won't help them, and won't help us," Mr Corbyn said as he dismissed claims immigration was having a negative impact on wages and public services.