Labour In for Britain chief Alan Johnson risked being accused of using inflammatory language as he branded Brexit campaigners as "extremists" on 10 May. The former cabinet minister defended his fiery put-down by claiming 'leave' supporters were entirely negative about the EU.

"They cannot find anything good to say and I think that's extreme," Johnson 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."

Johnson, a former postman who rose to the position of home secretary (2009-10), also defended David Cameron after the prime minister suggested that Brexit could trigger war on the continent.

"The Schuman declaration said 'we will make war between European nations, not just impartible, but unthinkable," Johnson told IBTimes UK. "Now the 'leave' side say 'ah, but war between European nations is unthinkable' without any sense of irony whatsoever. Once you think 'oh, now wars can't happen', I think David Cameron's point was very important."

The comments came after Johnson was joined in the summer rain by Labour leader Jeremy Corbyn, deputy leader Tom Watson and shadow minister for young people Gloria De Piero to launch Labour's pro-EU battle bus. The campaign vehicle will visit more than 100 towns and cities across the UK in the run-up to the 23 June referendum.

'Pollution knows no boundaries'

"I believe we have to vote to 'remain' in order to defend investment, defend jobs, and defend workers' rights. These things are absolutely crucial," Corbyn declared. "We face enormous challenges of climate change, of cybercrime, of the power of global corporations to try and evade taxes. We need to have a strong message that we can actually stand up to defend workers' rights, but also to deal with the huge challenges of global refugee movements."

"Pollution knows no boundaries, it doesn't respect national frontiers in anyway. Therefore, I'm supporting the 'remain' campaign in order to try and improve workers' rights, to stand up to global corporations and what they are doing to many people around this world."

The event comes after Boris Johnson hit out at out claims Brexit campaigners were "Little Englanders", as the former Mayor of London made a major speech on the EU referendum on 9 May. "I am a child of Europe. I am a liberal cosmopolitan and my family is a genetic UN peacekeeping force," Johnson joked.

"I can read novels in French and I can sing the Ode to Joy in German, and if they keep accusing me of being a Little Englander, I will. Both as editor of The Spectator and Mayor of London, I have promoted the teaching of modern European languages in our schools.

"So I find it offensive, insulting, irrelevant and positively cretinous to be told – sometimes by people who can barely speak a foreign language – that I belong to a group of small-minded xenophobes."