Chris Skidmore, who at one time was the government’s net zero tsar, accused Rishi Sunak of “siding with climate deniers”.
He announced in January that he was resigning the Tory whip and quitting as an MP - triggering a by-election which Labour won.
Writing in The Guardian today, Skidmore - who served as a minister under Theresa May and Boris Johnson - said Sunak had sought “division and polarisation” by trying to turn climate change into a culture war issue.
He said: “Worse still has been the rhetoric and extreme tone that has sought to politicise net zero as being forced upon people, a false narrative that is either the product of ignorance, or deliberate misinformation.”
Skidmore added: “For the first time, I cannot vote for a party that has boasted of new oil and gas licences in its manifesto or that now argues that net zero is a burden and not a benefit.
“Instead, like many others who know that we have neither choice nor any more time, and need to tackle the climate crisis now, I have decided that the Labour party is best placed to achieve economic growth and the green industrial revolution.
“Net zero is one of its five key priorities, and for this reason I will be voting Labour at this election.”
Skidmore’s announcement is yet another blow to Sunak and the Tories’ disastrous general election campaign.
He also follows another former Conservative MP, Mark Ogden, in announcing that he is now backing the Labour Party.