Britain’s new prime minister Rishi Sunak will now attend Cop27, after earlier saying he was too busy preparing for the upcoming budget.
Mr Sunak’s decision to miss the event, which is being held in Sharm El Sheikh in Egypt, was criticised by environmentalists as a “massive failure of leadership”.
The U-turn, his first following a series of policy reversals by predecessor Liz Truss, came after former prime minister Boris Johnson confirmed he will attend.
Mr Sunak will join other leaders, including US President Joe Biden and France's Emmanuel Macron, at the UN event, which begins on Sunday and runs until November 18.
He tweeted: “There is no long-term prosperity without action on climate change.
“There is no energy security without investing in renewables.
“That is why I will attend @COP27P next week: to deliver on Glasgow's legacy of building a secure and sustainable future.”
The prime minister took over last week at a time of crisis in British politics, after Mr Truss, resigned after 49 days in the job following resistance to her mini-budget, a series of U-turns and a lack of political and economic stability.
Mr Sunak initially said he was too busy to attend Cop27 because he was “focusing on the depressing domestic challenges we have with the economy”.
“I think that’s what people watching would reasonably expect me to be doing as well.”
Following that announcement, Nicola Sturgeon, First Minister of Scotland's devolved administration, said she would travel to Egypt for the conference.
Critics said Mr Sunak's absence would “make a mockery” of his government's commitment to tackle climate change.
On Tuesday, his spokesman said "significant progress" was being made on the financial statement.
Green MP Caroline Lucas welcomed the "screeching U-turn", calling his decision to stay away an "embarrassing mis-step on the world stage".
"Let this be a lesson to him — climate leadership matters," she added.
Deputy Labour leader Angela Rayner said the prime minister had been "dragged kicking and screaming into doing the right thing".
The Liberal Democrats claimed he was "only going after being embarrassed by Mr Johnson's attendance".
Alok Sharma, who was due to speak to British MPs for the last time as Cop26 president on Wednesday but could not because he was removed from the Cabinet, said he was “delighted” by the U-turn.
He earlier said he was “disappointed” by Mr Sunak's failure to attend.
“Delighted that @RishiSunak is attending #COP27 – completely agree with his comment that ‘there is no long-term prosperity without action on climate change’,” Mr Sharma tweeted.
As Cop26 questions began on Wednesday, Commons speaker Sir Lindsay Hoyle said Mr Sharma would no longer be fielding questions under Mr Sunak’s reshuffle.
Shadow climate change minister Kerry McCarthy responded: “I have to say it is disappointing that the Cop president [Mr Sharma] hasn’t been allowed to answer questions today.”
She added: “We have also just heard that the UK has failed to pay out more than $300 million promised at Cop to the green climate fund and the adaptation fund.
“Is that why the prime minister was trying to avoid going to Sharm El Sheikh? Because he is embarrassed that the UK has not delivered on all these promises it has made?”
Environment Secretary Therese Coffey replied: “I think she is being ungenerous. All of our pledges are still absolutely in place and she will recognise the work that this government and ministers are doing in order to bring partners together.”
So far, Downing Street's advice to King Charles III, a long-standing environmental campaigner, has been for him not to attend.
No 10 said last week that the position, first agreed under Mr Truss's leadership, still stood.
"As is standard practice, government advice was sought and provided under a previous PM, and it was unanimously agreed that it would not be the right occasion for the king to visit in person," a spokeswoman said.