The prime minister is due to use a speech on Wednesday afternoon to confirm plans to achieve net zero will be delayed.
A proposed ban on the sale of petrol and diesel cars, which was due to begin in 2030, is set to be pushed back to 2035. The phasing out of gas boilers is also set to be postponed.
It has triggered a backlash from several senior Conservatives who have warned the move will be bad for the environment, economy and hurt the party at the polls.
In a blistering intervention ahead of Sunak’s speech, Johnson said businesses needed “certainty about our net zero commitments”.
“This country leads on tackling climate change and in creating new green technology,” the former PM, an advocate of achieving net zero, said.
“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.”
Johnson added: “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.”
Among the Tories who savaged Sunak for the U-turn was Johnson ally Lord Goldsmith.
The former Foreign Office minister - who resigned from the government in June with an attack on Sunak for abandoning environmental policies - said: “We need an election. Now.”
“It says much about his low opinion of voters that he believes they will join him on the side of environmental destruction,” he said on Twitter.
Earlier today, car giant Ford issued a statement attacking Sunak for the decision to push back the ban on new petrol cars.