The 86-year-old grandee, who served under Margaret Thatcher and John Major, made the dramatic plea to die-hard Conservatives to “put the country first”.
Speaking alongside ousted Tories Dominic Grieve, David Gauke and Anne Milton – all standing as Independents – he said voters should back either them or the Lib Dems.
Asked what advice he would give to long-time Tory voters, Lord Heseltine said: “I’m telling them to vote for what they believe and what the Conservative Party has stood for all my life and certainly all of theirs – and to put country first.
“And what I think that means in practical terms is they either vote for the defrocked Conservative candidates, of which we have three excellent examples here, or they vote Lib Dem.”
It comes six months after Lord Heseltine lost the Tory whip for saying he would “lend” his own vote to the Lib Dems in the European Parliament elections.
His comments tonight went further than those he made in May, when he described his decision only as a personal “experiment”.
Boris Johnson was branded a power-hungry “spoilt child” as the four politicians addressed hundreds of disaffected voters in Gerrards Cross, Buckinghamshire.
Lord Heseltine told the crowd there was not a “shred of truth” to claims his support for rival candidates would put Jeremy Corbyn in Downing Street.
He claimed there was no way Mr Corbyn would win an outright majority for his “insane” manifesto and suggested he would not be Labour leader by Christmas.
The arch-Tory said fighting Brexit was more important to him than wiping out nationalisation, council houses and the Campaign for Nuclear Disarmament.
He said it was a “delusion” that Boris Johnson will “get Brexit done” by January 31 when he hasn’t even started talks over a UK-EU trade deal.
The grandee warned the Tories’ plans to create a Singapore-style tax haven after Brexit would be “wildly unpopular” – and warned voters not to trust Donald Trump.
Lord Heseltine said sarcastically: “Of course there is the great saviour – the ultimate justification – President Trump is on our side.
“And this most benign of American presidents, whose heart is there across the world to the underprivileged, fighting for his political life in an election period will break the habit of a lifetime and be generous to somebody else.
“If you believe that, you will believe… Brexit.”
He said Brexiteers “never tell you” what they will do once they have taken back control.
He added: “There is to the more cynical of us another explanation.
“That they actually know what they want to do would be wildly unpopular in this country because it affects the way we live and work, the way our companies operate.
“The health and safety, the environmental standards that patiently we have ourselves in Britain helped to create over the last 40 years.
“And the moment they start dismantling, creating a sort of Singapore this side of the Channel, then the reaction across the Conservative Party will be indelible and irradicable.”
Ex-ministers Mr Gauke, Mr Grieve and Ms Milton all ruled out going back to the Conservatives in the five-year Parliament if they defy the odds to win the election.
None of the three ruled out ever returning to the Tories – and Lord Heseltine said he is still a member.
But Mr Gauke said “I don’t see” the Tories returning to the One Nation Conservative party he joined – and warned it was part of a “substantial realignment” in British politics.
On Brexit, he added: “The middle ground is no longer there. It’s not going to come back.”
And Mr Grieve said: “My message is very simple – I’m sorry, my party’s gone bonkers”.
Ms Milton, the former skills minister who was one of 21 Tories ousted for blocking no-deal Brexit, launched a brutal personal attack on Boris Johnson’s character saying he “behaves like a spoilt child” and “threw his toys out the pram” by calling the election.
She said he is “seeking power and power alone”, adding: “I think he’s a very ill-mannered disrespectful man.
“And I think that does actually matter. His instinct is not good. He manages a party badly.
“His behaviour is absolutely disgraceful and I think my only regret is that he is not called out by the press more often for it.”
And Mr Gauke predicted Boris Johnson will stick to his deadline to end the Brexit transition period on December 31, 2020 – because otherwise he’d be ousted by hardline Tories.
Mr Gauke said being “captured” by the European Research Group would prevent the PM changing his mind.
“He won’t have the courage to do it and we will leave on WTO terms next year,” he said.
“And if that isn’t worth trying to stop, I don’t know what is.”