John Major has criticised the "squalid" campaign for Britain to quit the European Union.
The former Tory prime minister said he was "angry" about the "deceitful" claims being made by senior Brexit backers and accused them of "misleading" the public.
Mr Major said Mr Johnson, a former mayor of London, would not have the loyalty of Conservative MPs if he becomes party leader.
He also claimed the NHS would be "about as safe" in the hands of Mr Johnson, the Justice Secretary and former Cabinet minister Iain Duncan Smith as a "pet hamster would be with a hungry python".
Sir John told the BBC: "Firstly on the economy and what would happen if we actually left - the Leave campaign have said absolutely nothing to the British people and what they have said about leaving is fundamentally dishonest and it's dishonest about the cost of Europe.
"And on the subject that they have veered towards, having lost the economic argument, of immigration, I think their campaign is verging on the squalid."
He added: "I am angry at the way the British people are being misled.
"This is much more important than a general election. This is going to affect people, their livelihoods, their future, for a very long time to come and if they are given honest straightforward facts and they decide to leave, then that is the decision the British people take.
"But if they decide to leave on the basis of inaccurate information, inaccurate information known to be inaccurate, then I regard that as deceitful."
Mr Johnson said it was "absolute nonsense" he was backing Brexit because of personal leadership ambitions.
He said: "Obviously there is going to be a temptation by one side or the other to try to turn it into a personality driven conversation.
"My view about the EU has changed but that is because the EU has changed out of all recognition."
He said it was "frustrating" the Government had failed to meet its pledge to reduce migration to the tens of thousands and argued the Prime Minister "didn't get a sausage" from his renegotiations on Britain's relationship with Brussels.
