In a lecture at the Foreign Office in London, the former prime minister said the 2016 referendum vote was a “colossal misjudgement” that would leave the country poorer and weaker.
Speaking on the eve of a Brussels summit, at which Theresa May is battling to avoid a “no-deal” Brexit, Sir John said once people understood they had been deceived, those responsible would have “much to answer for”.
The former PM, who campaigned alongside Tony Blair close to the Irish border in the 2016 referendum, said: “We know the post-Brexit world will be very different from now.
“It cannot be otherwise, because no form of Brexit will remotely match up to the promises made by the Leave campaign in the referendum: they were vote-gathering fantasies, not serious politics.
“It will damage our national and personal wealth, and may seriously hamper our future security. It may even, over time, break up our United Kingdom. It will most definitely limit the prospects of our young.
“And – once this becomes clear – I believe those who promised what will never be delivered will have much to answer for. They persuaded and deceived the population to vote to be weaker and poorer.
“That will never be forgotten – nor forgiven.”
He denounced Mr Johnson, David Davis and other potential Tory leadership candidates – who he claims are undermining Mrs May and aiming to succeed her as prime minister – as “princelings”.
He said their “belligerent noises off” were making the PM’s task of negotiating a deal with Brussels even more difficult, while those whose focus was on self-advancement were “rarely the most suitable to be entrusted with power”.
He declared: “What government is not about is cheap grandstanding. It’s not about deceiving the electorate with slogans, or soundbites, or untruths or half-truths. It’s not about windy oratory that says nothing.
“And – most emphatically – it’s not about princelings fighting for the political crown of premiership. Coded messages that shriek ‘I’m the one’ are about as subtle as a punch on the nose.”
Sir John has form for attacking Mr Johnson. Two years ago, in a TV interview, he called him a “court jester” who had divided the Conservative Party which would never unite after his “squalid” Brexit campaign.
He also claimed the NHS, which Brexiteers said would receive £350m-a-week after Brexit, would be “about as safe” in the hands of Mr Johnson, Michael Gove and Iain Duncan Smith as a “pet hamster would be with a hungry python”.
And last month, campaigning in South Shields with David Miliband for a people’s vote, he said “routine attacks” on Theresa May’s leadership were “completely unacceptable” and he felt “even more closely drawn” to her when he looked at the alternatives.
In his Foreign Office speech, Sir John also voiced concern that as the extremes of right and left gained strength, the moderate “centre vote” would lose its traditional influence over policy.
“Our nation should not tolerate the unreasoning antipathy of the extremes – to the EU, to foreigners or to minority groups,” he said.
“Such antipathy is repellent, and diminishes us as a nation. Softer, more reasonable voices should not be drowned out by the raucous din of the loudest.
“I freely confess to a taste for compromise. I have always preferred good old British pragmatism to rigorous ideology. Politics is real life.
“It isn’t warfare. It isn’t a popularity contest. It’s about people. It’s about four nations who deserve more than an ideological tug-of-war.”