Former prime minister Sir John Major has publicly slammed the stance of those arguing for a British exit. Speaking on the BBC Andrew Marr show, Major said: "This is so important. For once, I am not prepared to give the benefit of the doubt to other people, I am going to say what I think. I think it is a deceitful campaign."

In a blunt statement, he added: "This is much more important than a general election. This is going to affect people, their livelihoods and their future for a very long time to come. If they are given honest, straightforward facts and they decide to leave then that is the decision the British people take. But if they decided to leave on the basis of inaccurate information, known to be inaccurate, then I regard that as deceitful."

Referencing the Leave campaign's tactics and claims by Boris Johnson and Michael Gove that Britain sends £350m a week ($508m) to the EU, Major indicated that the campaign was "verging on the squalid". He said: "I am angry at the way the British people are being misled."

But later in the show, Johnson defended the Brexit campaign, claiming the EU has "changed beyond recognition". He reacted to questions about his stance on immigration and whether he was really campaigning for the leadership of the Conservative Party.

Johnson sidestepped the attacks by Major, saying: "I think what people want to hear are the arguments and what we're setting out the Leave campaign, is an agenda for the government to take back control on 23 June on a lot of things that really matter to the people of this country."

On immigration, the former London mayor said: "For me this is a question of democracy, it's about public consent and explaining to people, at the moment we have absolutely no power to change our immigration policy."

Boris Johnson and John Major
Former British Prime Minister John Major (R) has slammed the tactics of Boris Johnson's (L) Vote Leave campaign Matt Cardy/Jeff Spicer/Stringer/Getty

In reference to previous election pledges to cut immigration, he said: "If you tell people you can cut immigration to the tens of thousands and if you then are unable – legally – to deliver what you have pledged because of EU, I think it is frustrating… all of us conservatives stood on that manifesto and thought we would get reform.

"We didn't get a sausage."

When pushed to respond to allegations that he was using the Leave campaign to bolster his own political career, Johnson claimed the notion was "nonsense".