Rishi Sunak’s climate credentials have come under question after backing out of attending the Cop27 climate summit days after giving the environment a smaller billing in his government.

The new prime minister has been accused of a “massive failure of leadership” after it was confirmed he will skip the United Nations conference in Egypt next month.

Liz Truss was set to attend the high-profile event in Sharm El-Sheikh but Downing Street said on Thursday that Sunak would instead focus on “pressing domestic commitments”.

No 10 also confirmed the demotion of the position of environment minister as Graham Stuart was reappointed to the role but stripped of his entitlement to attend cabinet.

Cop26 president Alok Sharma also lost his seat around Sunak’s cabinet this week.

Downing Street insisted the PM remains “absolutely committed” to supporting the climate conference hosted in Glasgow last year, denying he was downgrading the importance of tackling the climate crisis.

Last year when Sunak was chancellor, he arrived at the summit bearing a green version of the traditionally red budget briefcase wielded by the finance minister.

Labour’s shadow climate change secretary Ed Miliband said: “This is a massive failure of climate leadership. We were the Cop26 hosts and now the UK prime minister isn’t even bothering to turn up to Cop27.

“What Rishi Sunak obviously fails to understand is that tackling the climate crisis isn’t just about our reputation and standing abroad, but the opportunities for lower bills, jobs, and energy security it can deliver at home.”

Green MP Caroline Lucas said “shame on” Sunak, adding: “The first test of leadership is to turn up. The new PM’s decision not to attend Cop27 makes a mockery of any government claims on continued climate leadership – and what a shameful way to end the UK’s Cop presidency.”

Rebecca Newsom, the head of politics at Greenpeace UK, said the move suggests Sunak does not take climate change “seriously enough”.

“The UK Government is supposed to hand over the Cop presidency to their Egyptian counterparts at next month’s summit. 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,” she added.

A Downing Street spokeswoman said Sunak was focusing on domestic issues including preparations for the autumn budget, which has been delayed from Monday to November 17.

“The prime minister is not expected to attend Cop27 and this is due to other pressing domestic commitments including preparations for the autumn budget,” she told reporters.

She said the UK will be “fully represented” by Sharma and “other senior ministers”.

“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,” she said.

“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.”