The former prime minister dismissed Mr Johnson as a “court jester” and accused him of overseeing a “squalid” campaign that was deliberately misleading voters.
Accusing Mr Johnson of being responsible for divisions within the Tories, Sir John suggested that internal strife would continue long after the referendum if the former London mayor becomes leader of the party,
His comments in an interview on the Andrew Marr Show yesterday will pour fuel over an increasingly fiery struggle between senior Conservatives that threatens to overshadow the debate over EU membership.
Sir John, whose premiership was dogged by dissent from backbench eurosceptics, launched an assault on members of his own party saying he was “angry at the way British people are being misled”.
He also warned that in the event of a Brexit, “the Scots may hold another [independence] referendum. We might end up with Britain out of the European Union, and Scotland out of the UK”.
Sir John said: “I think this [Leave] is a deceitful campaign, and in terms of immigration, a really depressing and awful campaign. They are misleading people to an extraordinary extent.”
He poured particular scorn on claims from the Leave campaign that millions of migrants from Turkey could flood into the UK if the country is allowed to join the EU.
Sir John said: “Turkey will not be in the European Union for a very, very long time, if ever, and the Leave campaign know that. That’s the point – they know that.”
He also sought to cast doubt on claims by Mr Johnson and fellow Vote Leave figurehead, Michael Gove, that exiting the EU would allow £100 million more to be spent on the NHS every week, saying: “The NHS is about as safe with them as a pet hamster would be with a hungry python.”
In a clear warning to Mr Johnson – widely considered to have aspirations to succeed David Cameron as Tory leader – Sir John said: “I think I would offer him this piece of advice: if the Leave campaign led by Boris continue to divide the Conservative Party as they are doing at the present time, and if Boris has the laudable ambition, for it is a laudable ambition, to become prime minister, he will find if he achieves that he will not have the loyalty of the party he divided.
“Iain Duncan Smith was serially disloyal in the 1990s; when he became leader he was surprised that no-one was loyal to him, and Boris should learn from that.”
In interviews following Mr Major’s comments, Mr Johnson and Mr Gove sought to strike a more conciliatory tone, with the former London mayor saying it was “absolute nonsense” that he was campaigning for Brexit in order to advance his political career.
Mr Gove admitted the UK might still be a member of the EU by 2020 if the Leave campaign was successful, suggesting a lengthy period of adjustment and renegotiation would run into the next parliament.
Yesterday Mr Cameron also hit out, tweeting: “It’s becoming increasingly clear the Leave campaign don’t have a plan and are prepared to take a leap in the dark. We’re #StrongerIn the EU.”
However, in an interview in a Sunday newspaper, he attempted to dampen down tensions by suggesting both men would have roles in government after the referendum – but said he was “going to make [Vote Leave campaigners] pay” for their arguments in favour of leaving the EU.
Tories on the Leave side were quick to defend Mr Johnson, with MP David Davis saying Mr Major’s criticism was “harsh” and unwise.
Mr Davis told the BBC: “After 23 June we’ve got to pull the party together, make it work and with that sort of attack it’s a very bad idea.”
Armed forces minister Penny Mordaunt said: “The In campaign had an opportunity to properly engage in the debate and explain why the risks of remaining in the EU are worth it. Instead all they were able to offer was name-calling.”