Rishi Sunak has been accused of a "massive failure of leadership" after pulling out of attending a major climate summit in Egypt next month.
It comes just 24 hours after the UN warned the planet is heading for a "climate catastrophe" and urged world leaders not to put climate change on the "back burner".
Former Prime Minister Liz Truss had reportedly planned to attend November's summit in Sharm El-Sheikh, Egypt, where world leaders will meet.
Last year the UK hosted the gathering in Glasgow and countries pledged to revisit and strengthen their targets for limiting global warming to 1.5C.
But a No10 spokesperson said on Thursday: "The Prime Minister is not expected to attend Cop27 and this is due to other pressing domestic commitments including preparations for the Autumn Budget.
"The UK will be fully represented by other senior ministers as well as the Cop President Alok Sharma."
In his first reshuffle since entering No10, the new Prime Minister also kept both Mr Sharma and the climate minister Graham Stuart in their posts - but confirmed they will no longer attend Cabinet.
Downing Street insisted the PM remains "absolutely committed" to supporting the climate conference.
They denied that the decision to skip the global meeting signalled a downgrading of climate change as a priority.
The spokeswoman said: "It is a recognition of other pressing domestic commitments, not least preparations for the Autumn Budget."
She said: "We remain committed to net zero and to leading international and domestic action to tackle climate change. The UK is forging ahead of many other countries on net zero.
"We will obviously continue to work closely with Egypt as the hosts of Cop27 and to make sure that all countries are making progress on the historic commitments they made at the Glasgow climate pact."
But Labour's Shadow Business Secretary Ed Miliband blasted: "This is a massive failure of leadership.
"We were the COP26 hosts and now the UK PM will be missing in action.
"What Rishi Sunak fails to understand is that tackling the climate crisis is key to our domestic priorities, including how we get to lower bills, jobs, and energy security."
Head of politics at Greenpeace UK Rebecca Newsom added: "For Rishi Sunak not to show up is like a runner failing to turn up with the baton at a crucial stage of the relay.
"Coming just after the ousting of Alok Sharma from the cabinet, this suggests that the new Prime Minister neither takes the climate crisis seriously enough, nor recognises the opportunities for Britain to take a leadership role in helping to solve it."
Speaking to the BBC on Wednesday the UN Secretary General Antonio Guterres urged countries to "bring back climate change to the centre of the international debate".
"There has been a tendency to put climate change on the back burner," he said. "If we are not able to reverse the present trend, we will be doomed."
