David Cameron has said he wants to support Prime Minister Rishi Sunak "at a hard time", after making a dramatic comeback to government in a major cabinet reshuffle.
The former prime minister has been appointed foreign secretary and accepted a peerage to take the post.
He replaced James Cleverly, who became home secretary after Mr Sunak sacked Suella Braverman.
Lord Cameron admitted it was "not usual" for a former PM "to come back".
But he said at a time when the country faced "daunting challenges" in the Middle East and Ukraine, he hoped his experience would be helpful to Mr Sunak's government.
"I've decided to join this team because I believe Rishi Sunak is a good prime minister doing a difficult job at a hard time," Lord Cameron said. "I want to support him."
Later the Foreign Office said Mr Cameron had spoken to US Secretary of State Antony Blinken on Monday evening, and they discussed "the conflict in the Middle East, Israel's right to self defence and the need for humanitarian pauses to allow the safe passage of aid into Gaza" - as well as their continued support for Ukraine and the strength and depth of the relationship between the UK and the US.
Mrs Braverman's sacking kickstarted Monday's cabinet reshuffle by Mr Sunak, whose party is lagging far behind Labour in opinion polls, after more than 13 years in power.
Mr Sunak's decision to sack Mrs Braverman came after the former home secretary accused the Metropolitan Police of bias in its handling of protests.
The prime minister's spokesperson stressed the importance of having a "united team" and acknowledged there had been "differences of style" between Mrs Braverman and Mr Sunak.
In a speech at the Lord Mayor's Banquet in London, Mr Sunak said the world faced "deeply challenging times" and "it falls to us to do everything we can to shape these events".
He said the UK government had "delivered one of the most significant years for British foreign policy in recent times" and praised Mr Cleverly for his work on Ukraine as foreign secretary.
"I'm pleased to have appointed a new foreign secretary who will build on everything we have achieved in the last year," Mr Sunak said.
By bringing back Lord Cameron and firing Mrs Braverman, who is popular on the right of the Conservative Party, the prime minister has risked deepening divisions among his MPs.
Conservative former Cabinet minister Sir Jacob Rees-Mogg said the Conservatives were "in danger of losing votes to the Reform party".
Reform leader Richard Tice was "as happy as can be" when he saw him earlier, he told BBC Newsnight, adding: "The Champagne will be flowing in the Reform party headquarters tonight after what's been done today."
In key changes, Steve Barclay took Therese Coffey's job as environment secretary, and Victoria Atkins became health secretary.
Former transport minister Richard Holden became Tory party chairman, taking over from Greg Hands.
Other senior cabinet members remained in post, including Chancellor Jeremy Hunt, Defence Secretary Grant Shapps and Education Secretary Gillian Keegan.
It was Lord Cameron's appointment that stunned Westminster and made him the first former prime minister to re-enter government since the 1970s.
The surprise move marked an unexpected return to frontline politics - seven years on from Lord Cameron's resignation as prime minister after the UK voted to leave the European Union in 2016.
Lord Cameron, who had campaigned to remain in the EU, has kept a fairly low profile since leaving politics, but lamented "my political career ending so fast" in his 2019 memoir.
The former prime minister has been critical of Mr Sunak's government, notably his decision to scrap the northern leg of the HS2 rail link and cutting the UK's aid budget.
And in his speech to the Tory conference, Mr Sunak sought to distance himself from his predecessors and sold himself as a "change" prime minister.
Lord Cameron said although he had "disagreed with some individual decisions" by Mr Sunak's government, "politics is a team enterprise".
"I'm a member of the team and I accept the cabinet collective responsibility that comes with that," Lord Cameron said.
The return of Lord Cameron has been welcomed by centrist Tory MPs, but derided by Brexit backers on the right of the party.
The Liberal Democrats are calling for Lord Cameron's peerage to be blocked, referring to his lobbying for collapsed finance company Greensill Capital.
Senior Labour MP Pat McFadden said Lord Cameron's appointment "puts to bed the prime minister's laughable claim to offer change from 13 years of Tory failure".
Lord Cameron said he had resigned from the various business and charitable roles - including president of the Alzheimer's Society - he had held since quitting as prime minister.
"I have one job - to be foreign secretary and work with the prime minister for the UK to be as secure and prosperous as possible in a difficult and dangerous world," he said.
The foreign secretary insisted the Greensill affair was "in the past" and had been "dealt with".
The ministerial reshuffle followed growing criticism of Mrs Braverman over her remarks about policing ahead of a pro-Palestinian march in London over the weekend.
Mrs Braverman defied the PM last week in an unauthorised article accusing police of "double standards" at protests, claiming right-wing protesters were "rightly met with a stern response", while "pro-Palestinian mobs" were "largely ignored".
Labour and other opposition parties accused Mrs Braverman of inflaming tensions between the pro-Palestinian march and far-right counter protesters on Saturday, when nearly 150 people were arrested.
Transport minister Huw Merriman said the party had had "a difficult week in terms of some of the comments", arguing Mr Sunak's refreshed cabinet would "appeal" to the country.
But the former Tory treasurer Lord Cruddas criticised Mr Sunak's reset, branding it a "coup" by those who advocated remaining in the EU.
The reshuffle means that for the first time since the Conservatives won the 2010 election, there are no women holding any of the four of the most senior positions in cabinet.
In other changes:
Laura Trott was promoted to Treasury Chief Secretary
Former chief secretary John Glen became Paymaster General
Mr Hands was appointed a minister in the Department for Business and Trade
Former work and pensions secretary Esther McVey was brought back as a Cabinet Office minister
Lee Rowley has become housing minister, replacing Rachel Maclean