Donald Trump has described Rishi Sunak's U-turns on a raft of net zero measures as "smart", insisting the climate crisis is a "green new hoax".
It comes days after the prime minister confirmed he would delay a ban on the sale of new diesel and petrol cars by five years and weakened targets to phase out gas boilers.
The former US president posted on his own social media platform Truth Social: "Prime Minister Sunak of the United Kingdom has very substantially rolled back the ridiculous "Climate Mandates" that the United States is pushing on everyone, especially itself.
"I always knew Sunak was smart, that he wasn't going to destroy and bankrupt his nation for fake climate alarmists that don't have a clue.
"In the meantime the US keeps rolling merrily along, spending Trillions of Dollars trying to do that which is not doable, while at the same time breathing in the filthy and totally untreated air floating over our once great Country from China, India, Russia, and Parts Unknown.
"They are all building Coal Fired Plants by the hundreds each year, and Germany, which has almost destroyed itself with its ridiculous form of the Green New Hoax, has just joined in.
"Congratulations to Prime Minister Sunak for recognizing this SCAM before it was too late! The Green New Hoax will take down the US, perhaps even sooner than our Open Border of Death. IT MUST BE STOPPED. MAGA!!!"
Mr Sunak insisted he is still committed to reaching net zero by 2050, but that the transition can be done in a "fairer and better way".
"Our destiny can be of our own choosing," Mr Sunak said - while calling for politicians to be "honest" about the costs of green policies on families.
However, shadow net zero secretary Ed Milliband said the prime minister "does not give a damn" about climate change.
Speaking to the online conference of Labour's sister Co-operative Party, the former leader added: "I think Rishi Sunak simply sees the climate crisis as an obligation to be managed, not an opportunity to be seized, and we see it as an opportunity to be seized."