Cameron replaced former Foreign Secretary James Cleverly, who in turn replaced Suella Braverman after she was sacked as home secretary.
Braverman caused controversy last week after penning an article in The Times in which she described "pro-Palestinian mobs" as "disturbingly reminscent" of scenes during the height of the conflict in Northern Ireland.
Her comments have been criticized as having stoked tension ahead of a pro-Palestinian march in London on Saturday, November 11, the UK's Remembrance Day, which also saw violent scenes involving far-right protesters.
What did Cameron say?
The former prime minister recently made headlines when he publicly criticized Sunak's decision to scrap the Birmingham-Manchester leg of a high-speed rail project known as HS2.
"Though I may have disagreed with some individual decisions, it is clear to me that Rishi Sunak is a strong and capable Prime Minister, who is showing exemplary leadership at a difficult time," Cameron wrote on X, formerly known as Twitter, confirming his appointment.
"We are facing a daunting set of international challenges, including the war in Ukraine and the crisis in the Middle East. At this time of profound global change, it has rarely been more important for this country to stand by our allies, strengthen our partnerships and make sure our voice is heard."