Liz Truss will accompany King Charles III on his tour of the UK next week as he leads the nation in the “significant moment” of mourning for the late Queen.
The Conservative prime minister and the King will head to Scotland on Monday morning, followed by visits to Northern Ireland on Tuesday and Wales on Friday.
Ms Truss will support Charles by joining him at “services of reflection” around the country during the 10-day mourning period, Downing Street has said.
The prime minister’s official spokesman said it was not a formal or constitutional role, but was agreed by No 10 and Buckingham Palace.
“It’s not a requirement, but the prime minister believes it’s important to be present for what is a significant moment of national mourning around the United Kingdom,“ the No 10 spokesman said.
Ms Truss will also be present when Charles attends parliament on Monday morning for the presentation of addresses ceremony with MPs, peers and members from the legislatures of Scotland, Wales and Northern Ireland.
The new PM, her ministers, opposition party leaders and other senior figures, took an oath of allegiance to the new King as parliament met for a rare Saturday sitting.
The King also held an audience with the new prime minister and her cabinet, followed by a separate audience with Sir Keir Starmer, Sir Ed and the SNP’s Westminster leader Ian Blackford.
Meanwhile, former Labour prime minister Gordon Brown said Britain could become more like a “Scandinavian monarchy” – saying he expected Charles to perform his duties in a more informal way as part of a slimmed-down royal family.
Praising the Queen, Mr Brown told the BBC’s Laura Kuenssberg “she actually knew better about what was happening to the country than I was – was quite embarrassing.”
The former PM said could have been “in endless debates or cabinet meetings or something” while he said the Queen had been keeping herself up to date by watching television and would be “getting notes from her secretaries”.
David Cameron revealed how he had to apologise to the Queen after microphones picked up the then PM recounting how the Queen “purred” down the telephone when he informed her of the result of Scotland’s 2014 referendum.
Mr Cameron told the BBC’s Sunday with Laura Kuenssberg programme: “It was a very upfront and fulsome apology done very quickly at the beginning of an audience. I think that is all I should say.”
Mr Cameron also said he thought it was “entirely right” for Charles to have written to ministers with his views when he was Prince of Wales.
The former PM said the so-called “spider memos” of letters to ministers on various issues from herbal medicine to badger culls – released in 2015 after a lengthy legal battle – should have stayed private.
“I never felt he tried to influence me improperly in any way,” the former PM said. “I think the heir to the throne has a perfect right to have interest in issues like the environment, preserving wildlife, his interest in the built environment.”
He added: “I think it is entirely right that the heir to the throne can discuss things with politicians. Why not? I don’t think there should be any public concern about that and my view is the letters should have remained private.”
Mr Cameron said the new King would be a “brilliant” monarch after serving the “longest apprenticeship in history” – revealing that he held secret audiences with Charles to help him prepare for his regular auidences with the PM.
Former Tory MP Sir Nicholas Soames, the grandson of Winston Churchill and a close friend of Charles, said: “Clearly he will never get involved in any form of political controversy.”
