Mr Johnson said Britain “cannot afford to falter now or in any way lose our ambition for this country” as he warned the PM of rowing back on green policies.
And former Tory environment minister Zac Goldsmith called for a general election “now”, after warning the PM was “dismantling Britain’s credibility” on tackling climate change.
The dramatic interventions came as Mr Sunak is expected to use a speech this afternoon to announce a major shift in the party’s approach to green policy, saying he wants to achieve net zero in a more “proportionate way”.
It comes after the BBC revealed changes could be made to as many as seven core commitments, including weakening the plan to phase out gas boilers from 2035 and delaying the ban on the sale of new petrol and diesel cars – currently due to come into force in 2030 because of Mr Johnson – by five years.
As Mr Sunak called an emergency cabinet meeting ahead of a 4.30pm press conference to set out his plans:
Business chiefs lashed out at the PM, accusing him of compromising climate commitments for “short-term political gain”
One of the chancellor’s own economic advisors warned Mr Sunak’s plans would be “bad for the UK”
Tory MPs attacked the plans, urging Mr Sunak to be “exceptionally careful of seeking to extract political advantage” on green issues
Tory former environment minister Lord Deben attacked the PM’s “stupid” watering down of climate pledges, warning the move will be “extremely damaging” and may face a legal challenge
Another former Tory environment minister Lord Goldsmith called for a general election “now”, warning the PM was “dismantling Britain’s credibility” on climate change
Environmental campaigner and former US vice-president Al Gore said it is “unfortunate” and Mr Sunak is “doing the wrong thing”
Mr Johnson said: "Business must have certainty about our net zero commitments.
"This country leads on tackling climate change and in creating new green technology. The green industrial revolution is already generating huge numbers of high quality jobs and helping to drive growth and level up our country.
"Business and industry - such as motor manufacturing - are rightly making vast investments in these new technologies.
"It is those investments that will produce a low carbon future - at lower costs for British families.
"It is crucial that we give those businesses confidence that government is still committed to net zero and can see the way ahead.
"We cannot afford to falter now or in any way lose our ambition for this country."
But Sir Jacob Rees-Mogg, who served in Mr Johnson’s Cabinet, branded the former prime minister a “net zero zealot” as he backed Mr Sunak’s moves to water down green plans.
He told BBC Radio 4’s World At One programme: “I’ve never been as much of a net zero zealot as Boris is. I mean, he genuinely believes in a more high wire approach in this area.
“I like to have a safety net under any high wire and I think what the Government’s doing now is using the safety net. And I think it’s absolutely right. I could not be more supportive of what the Prime Minister is doing under these circumstances.”
He added that “we need intelligent net zero”, meaning “getting to it in a way that people can afford and that doesn’t harm our industry”.
Lord Goldsmith, who quit in June with a swipe at Mr Sunak’s “apathy” toward climate change, claimed to have received hundreds of messages from Conservative friends in government, parliament and around the world telling him the PM’s plans vindicate his decision.
“I didn’t want vindication. I hoped it would add pressure on govt to prove me and others wrong. We need an election. Now,” he said.
Earlier Lord Goldsmith said net zero is “one of the few areas where the UK is really looked up to”.
But he added: “Today Mr Sunak is dismantling that credibility, not by accident but by choice.”
Lord Goldsmith questioned the PM’s mandate to tinker with the government’s net zero policies, saying he took over a party whose last election manifesto “could not have been clearer about our commitment to tackle climate change”.
“His short stint as PM will be remembered as the moment the UK turned its back on the world and on future generations. A moment of shame,” he added.
Environmental activist and former US vice president Al Gore said: “I think it is unfortunate that he would do that, I think the people of the UK largely agree that it is the wrong decision.
“That is for the people of the UK to address, and I certainly disagree with him.
“He is doing the wrong thing.”