Queen Elizabeth II used her Platinum Jubilee - marking a record 70 years on the British throne - to state that Camilla, the wife of Prince Charles, should have the title queen consort when the time comes for Charles to take the throne.
The move shapes the future of the monarchy on her historic milestone.
In what observers called a "hugely significant intervention," the monarch expressed her "sincere wish" that Camilla will be known as Queen Consort when Charles, also known as the Prince of Wales, becomes King.
As she reached her 70th year on the throne, Elizabeth II, in the twilight years of her reign, set out her hopes for her daughter-in-law, once a royal mistress and now certain to be called Queen and crowned at Charles's side.
Elizabeth II, using the rarest of Jubilees to put her affairs in order on a public platform, also shared her succession support for her eldest son as monarch.
The Queen, 95, said in the written message: "I would like to express my thanks to you all for your support. I remain eternally grateful for, and humbled by, the loyalty and affection that you continue to give me."
"And when, in the fullness of time, my son Charles becomes King, I know you will give him and his wife Camilla the same support that you have given me; and it is my sincere wish that, when that time comes, Camilla will be known as Queen Consort as she continues her own loyal service."
Queen Elizabeth II attends a reception in the Ballroom of Sandringham House, which is the Queen's Norfolk residence, to celebrate the start of the Platinum Jubilee. Photo: Joe Giddens.
The 73-year-old, and Camilla, is Duchess of Cornwall, were "touched and honoured" by the Queen's gesture, Clarence House said.
The prince will make his own public declaration on Sunday celebrating his mother's Jubilee.
A spokesman for the couple said: "The Prince of Wales will be issuing a statement of congratulation to the Queen on Accession Day."
"He and the Duchess of Cornwall are touched and honoured by Her Majesty's words."
A lifelong duty
The Queen who acceded to the throne on February 6, 1952 on the death of her father George VI, also renewed her lifelong pledge of duty, despite approaching her 96th birthday this year.
"As we mark this anniversary, it gives me pleasure to renew to you the pledge I gave in 1947 that my life will always be devoted to your service," she said.
She also paid tribute to the her late husband, the Duke of Edinburgh, for the sacrifices he made as consort, thanked her family for their support, and made reference to the Queen Mother's role as a Queen Consort.
Camilla will automatically take the same role as consort when Charles becomes king.
The Queen wrote: "I am fortunate to have had the steadfast and loving support of my family. I was blessed that in Prince Philip I had a partner willing to carry out the role of consort and unselfishly make the sacrifices that go with it."
"It is a role I saw my own mother perform during my father's reign."
Camilla would technically have been Queen when Charles acceded to the throne, and only legislation would have prevented it.
But royal aides insisted, when she married Charles in 2005, that Camilla did not want to be queen and said originally that the former Mrs Parker Bowles "intended" to be known instead as Princess Consort - the first in British history - instead.
But the careful use of the verb "to intend" left this open to change in the future.
Any mention of "Princess Consort" was removed from Charles's website during a revamp in 2018.
Marriage to Diana
Much has changed in the years since Charles, who aides once said had no intention of remarrying, wed his long-term love.
Camilla was blamed for the breakdown of the prince's marriage to Diana, Princess of Wales, and when news of their affair first came to light, she faced vitriolic criticism.
Yet now, with the Queen setting out her wishes and calling for people to support Charles and Camilla, the duchess will undoubtedly be crowned alongside the prince at his coronation when the time comes.
Former BBC royal correspondent Peter Hunt said: "This is the most extraordinary message. The Queen is ensuring the transition, when it comes, to her son as king is as seamless and trouble free as possible."
"She's future-proofing an institution she's served for seventy years. And for Camilla, the journey from being the third person in a marriage to queen-in-waiting, is complete."
On the eve of her Platinum Jubilee, the Queen was on "sparkling" form as she laughed and joked at a celebratory reception at Sandringham House.
The head of state cut a Jubilee cake, met members of the local Women's Institute and chatted to former cookery school student Angela Wood who helped to perfect the famous coronation chicken dish served to guests after her 1953 Coronation ceremony.
In her message, the Queen also reflected on the sad anniversary of the death of her father.
She spoke of looking forward to the rest of her Jubilee year, writing: "As I look ahead with a sense of hope and optimism to the year of my Platinum Jubilee, I am reminded of how much we can be thankful for."
"These last seven decades have seen extraordinary progress socially, technologically and culturally that have benefitted us all; and I am confident that the future will offer similar opportunities to us and especially to the younger generations in the United Kingdom and throughout the Commonwealth."
She acknowledged the challenges faced by the nation during the pandemic, saying she hoped the Jubilee would "bring together families and friends, neighbours and communities - after some difficult times for so many of us."
