The Princess Royal will take on the ceremonial role of the King’s “protector” during the first state opening of Parliament of his reign.
Charles, 74, has asked his sister to ride in the procession as Gold Stick In Waiting, a key role historically handed to a person entrusted with the personal safety of the monarch.
Princess Anne, 73, carried out the same role during the Coronation in May, riding on horseback from Westminster Abbey to Buckingham Palace.
On Tuesday, she will travel in a carriage behind the King and Queen, as the grand procession makes its way to the Houses of Parliament.
She will then enter the chamber alongside the King, the first time she is thought to have accompanied the monarch since 1985 when she joined her late mother, Elizabeth II.
Charles will preside over the first full ceremonial state opening since 2019.
The event, known for its pomp and pageantry, was cancelled in 2020 and scaled back in 2021 due to the Covid pandemic. Last year, Charles and Prince William stood in for the ailing Queen as counsellors of state during a reduced state ceremonial.
On Tuesday, as tradition is restored, the King is expected to wear the heavy Imperial State Crown, which his mother last wore for the 2016 state opening. After that, due to its weight, it was instead carried on a cushion and placed on a table nearby.
The King will reportedly travel to the Houses of Parliament in the ornate Diamond Jubilee State Coach, escorted by members of the Household Cavalry Mounted Regiment. He will be preceded by a coach carrying the royal regalia - the crown, the Cap of Maintenance and Sword of State.
During the ceremony, he will wear the Robe of State, as per tradition.
Meanwhile, the Queen is expected to give a nod to her late mother-in-law by wearing her diamond Diadem.
The late Queen wore the £6 million crown for her coronation in 1953 and every state opening since. With more than 1,300 diamonds, the Diadem, a familiar sight from stamps and coins, was made for George IV in 1820.
For the first time in four years, the chamber will be packed with 600 guests, ready to listen to the first King’s Speech in more than 70 years, which will be delivered by the monarch from the throne in the House of Lords before he returns to Buckingham Palace in a grand carriage procession.
The King is expected to pay tribute to his mother when he delivers his speech, following a precedent set by his predecessors.
When Elizabeth II gave her first state opening speech on November 4 1952, she paid tribute to her father’s “selfless devotion to his duties” and the example he set. She said it would be her “constant endeavour to follow” that standard.
George V, who delivered his first state opening speech on February 6 1911, said:
“In opening the first Parliament elected in my reign, the grievous loss which the Empire has sustained by the death of my beloved father is uppermost in my thoughts.”