LONDON — When Prime Minister Rishi Sunak of Britain travels to the Egyptian seaside resort of Sharm el Sheikh next Monday, he will share the spotlight with his former boss and sometime nemesis, Boris Johnson, who plans to attend the United Nations’ climate-change conference there as a private citizen.
That was not supposed to happen. Mr. Sunak first said he would skip the meeting to stay in London to oversee a new fiscal plan being rolled out on Nov. 17. But after Mr. Johnson declared his intention to go, the new prime minister reversed course and said he would travel to Egypt after all.
The two men will move in different orbits: Mr. Sunak jetting in for a day to meet other world leaders; Mr. Johnson giving interviews and making the rounds as a celebrity attendee.
Still, it is a harbinger of the kind of disturbance Mr. Johnson may continue to cause in Mr. Sunak’s political weather system: popping up unexpectedly, grabbing attention, disrupting his successor’s plans and serving as a reminder of a time when Britain’s political chatter was dominated by illicit parties at Downing Street rather than a painful cost-of-living crisis.
“Even if he doesn’t mean to be unhelpful, it is going to be newsworthy any time he says anything different to government policy,” said Gavin Barwell, who served as chief of staff to Prime Minister Theresa May. “From the reports of his recent meeting with M.P.’s, he intends to defend what he sees as key elements of his legacy.”
This situation is not unique to Mr. Sunak. Mr. Barwell dealt with Mr. Johnson’s outsize personality when he served as Mrs. May’s foreign secretary. He predicted that Mr. Johnson would haunt Mr. Sunak’s predecessor, Liz Truss, like Banquo’s ghost tormented Shakespeare’s Macbeth — a prediction that did not come to pass, if only because Ms. Truss served a mere 50 days.
When Mr. Sunak replaced her late last month, he praised Mr. Johnson for his “warmth and generosity of spirit.” But their relationship is fraught. It was Mr. Sunak’s resignation as Mr. Johnson’s chancellor of the Exchequer last July that triggered his boss’s downfall. And when Mr. Sunak declared his candidacy for Conservative Party leader after Ms. Truss’s resignation, it was Mr. Johnson who briefly threatened to derail Mr. Sunak by running himself.
Last week, Mr. Johnson gathered Conservative lawmakers to thank them for supporting that abortive bid, according to Politico. While he told them he would back Mr. Sunak, he also said he would defend what he viewed as his legacy achievements: Brexit, the party’s 2019 election victory and Britain’s support for Ukraine.
There is no evidence that Mr. Sunak plans to dismantle any of them. He placed his first call as leader to President Volodymyr Zelensky of Ukraine, pledging Britain’s “steadfast support” in the war against Russia. He reappointed the defense secretary, Ben Wallace, who has spearheaded that support, even though Mr. Wallace supported Mr. Johnson in his bid.
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Mr. Sunak also kept on Ms. Truss’s foreign secretary, James Cleverly, another supporter of Mr. Johnson, who, like Mr. Wallace, is a familiar figure to the Biden administration — in this case, because Mr. Cleverly focused on the United States in a previous stint as a minister in the Foreign Office.
The White House has been reassured by these personnel decisions, according to a senior administration official, because they have lent continuity to the trans-Atlantic relationship despite the political upheaval in London. Britain’s top foreign-policy officials have served the past three prime ministers, a contrast to other cabinet posts, like chancellor, which have been a revolving door.
Mr. Wallace, a former British Army officer, is a conspicuous symbol of Mr. Johnson’s Ukraine legacy. He has recently been involved in sensitive discussions at the Pentagon, including reports that Russia was weighing the use of a nuclear weapon in Ukraine. At home, he has campaigned aggressively for Britain to increase its military spending, despite an estimated 40 billion pound ($44 billion) fiscal hole that Mr. Sunak has warned is going to require spending cuts across the government.
Mr. Sunak has pointedly refused to embrace Ms. Truss’s pledge to hike military spending to 3 percent of Britain’s gross domestic product by 2028 — up from slightly more than 2 percent now. Mr. Wallace, testifying before a parliamentary committee on Wednesday, conceded that the goal had become “aspirational.”
“I will fight every bit of the way to see what I can get,” Mr. Wallace said, though he denied reports that he had threatened to resign if the government did not honor the 3 percent goal. On Thursday, Mr. Wallace met with the current chancellor, Jeremy Hunt, to make a case for his department’s budget.
Budget pressures in Europe and the United States are likely to bear on military support for Ukraine over the coming months. But there was no indication that Britain planned to scale back its deliveries of weapons or other support for Ukraine, according to the senior administration official — a message that Mr. Sunak conveyed when President Biden gave him a congratulatory call last week.
Mr. Johnson’s legacy on climate is respectable, if not as notable as his support for Ukraine. He played host at the last United Nations meeting, in Glasgow, which produced an agreement to urge rich nations to “at least double” funding by 2025 to protect the most vulnerable nations from a hotter planet. But it did not resolve how much and how quickly each nation should cut its emissions over the next decade.
Mr. Sunak, though a supporter of Britain’s net-zero goals, has sent other signals that suggest he is less than fervent about the issue. He demoted Alok Sharma, Mr. Johnson’s chief climate negotiator, from the cabinet. Mr. Sharma won praise for his management of the Glasgow meeting, known as COP26.
“There is no long-term prosperity without action on climate change,” Mr. Sunak said on Twitter, announcing his change of heart about going to the conference. “That is why I will attend Cop27 next week: to deliver on Glasgow’s legacy of building a secure and sustainable future.”
(Among the sessions in which Mr. Johnson will take part at the conference is an interview with The New York Times at its event on the sidelines. Mr. Sunak has also been invited, but has not yet responded.)
His reversal on attending the conference drew praise from various quarters, including Mr. Johnson’s father, Stanley Johnson, a former member of the European Parliament who has long been active in environmental issues. The elder Mr. Johnson, speaking to the Foreign Press Association on Thursday, added: “I’m pleased that Boris is going to COP. Everybody is pleased about that.”
Not everybody: Some climate experts said Mr. Johnson’s decision had put the prime minister in a no-win position.
“It doesn’t help because it underscores the political mistake that Rishi Sunak made,” said Tom Burke, the chairman of E3G, an environmental research institute. “Deciding not to go and announcing that he wasn’t going was a mistake. This just draws attention to Britain’s lack of global leadership.”