On Thursday No 10 said Mr Sunak was not expected to attend "due to other pressing domestic commitments".

But on Monday the prime minister's official spokesman said this position was "under review".

Alok Sharma, the UK's COP26 president, is among those saying the PM should go.

The UK is the current holder of the COP presidency, after hosting the summit in Glasgow last year.

The annual UN climate summits are designed to help governments agree steps to limit global temperature rises.

Mr Sharma will hand over the presidency to Egypt at the COP27 summit in Sharm el-Sheik, which takes place from 6 to 18 November.

The conference finishes the day after Chancellor Jeremy Hunt is due to set out the UK's tax and spending plans in his highly anticipated autumn statement.

On Monday, Mr Sunak's official spokesman said: "The prime minister is focused on pressing domestic issues, most significantly preparing for the autumn statement, so any attendance at Cop would depend on progress on preparation for that fiscal event, and that work is ongoing."

"The prime minister fully recognises the importance of the COP summit and is fully committed to addressing climate change," he added.

Asked on BBC Breakfast whether the prime minister could attend, Environment Minister Mark Spencer said Mr Sunak had "a huge inbox", with challenges including the economy and rising global energy and food prices.

"His focus at the moment is dealing with the autumn statement and the government's response to those global challenges," he said.

Mr Spencer added: "I'm sure if his diary allows he would want to go but at this moment in time don't quite know if he's going to be have time to do that."

Mr Sunak has faced criticism from opposition parties, environmental groups and some Conservatives, after No 10 said he was not expected to attend the summit.

Liberal Democrat climate spokeswoman Wera Hobhouse called for Mr Sunak to "immediately confirm his attendance", adding: "It shouldn't take Boris Johnson going to COP to embarrass Rishi Sunak into doing the right thing."

There are reports Mr Johnson, who attended the Glasgow summit when he was prime minister, could go to Egypt for this year's event.

Labour have accused Mr Sunak of a "failure of leadership" and Green Party MP Caroline Lucas said it was "absolutely so wrong" that Mr Sunak was not going, as the UK is still COP president, adding: "Symbols matter."

At the weekend Alok Sharma, who was recently demoted from cabinet, told the Sunday Times he was "pretty disappointed" at news Mr Sunak was not going, saying his attendance would signal the UK's "renewed commitment on this issue".

On Monday, the government's most senior environmental advisers, including influential climate experts Lord Stern and Laurence Tubiana, urged Mr Sunak to attend the conference, saying it presented "an opportunity... to restore the trust and confidence of the international community in Global Britain".

US President Joe Biden, France's Emmanuel Macron and Scottish First Minister Nicola Sturgeon are all due to attend, while Mr Sunak's predecessor Liz Truss had been set to go when she was PM.

King Charles, who is a longstanding champion of environmental issues, will not attend after Buckingham Palace sought advice from then-PM Ms Truss and agreed he would not go in person. The advice has not changed under new PM Mr Sunak.

However, the monarch will host a reception at Buckingham Palace on 4 November, on the eve of the conference, for 200 international business leaders, decision makers and charities to mark the end of the UK's COP presidency and look ahead to the summit in Egypt.

Mr Sunak will "say a few words" at the event, which will also be attended by US climate envoy John Kerry.

Teenage climate activist Greta Thunberg, who has said she will not be at the summit in Egypt, called it a "scam" and said it was "symbolic" the conference was being held in a country which "violates many basic human rights".

"Many world leaders are too busy to go there because they have their own problems. With that mindset we're not going to be able to solve many of the problems that we face," she told an event in London on Sunday.

The Egypt conference is expected to focus on three main areas - reducing emissions, helping countries prepare for and deal with climate change, and securing technical support for developing countries for these activities.