Queen Elizabeth II on Sunday became the first British monarch to reign for seven decades, announcing her "sincere wish" that Camilla, the wife of her heir Prince Charles, should ultimately be known as Queen Consort.

Britain's longest-serving monarch acceded to the throne aged 25 on February 6, 1952, following the death of her father King George VI.

She marked the historic date quietly at Sandringham, her estate in eastern England where her father died.

Queen Elizabeth II on Sunday became the first British monarch to reign for 70 years
Queen Elizabeth II on Sunday became the first British monarch to reign for 70 years POOL via AFP / Steve Parsons

But in a major statement on the future of the royal family, the 95-year-old released a message to the nation, saying "it is my sincere wish that, when that time comes, Camilla will be known as Queen Consort".

This means Camilla, now 74, would be crowned alongside Charles, now 73, and known to the public as Queen Camilla, royal experts said.

Charles said the couple were "deeply conscious of the honour represented by my mother's wish", which would accord Camilla the full title of a monarch's wife.

He praised Camilla, saying: "my darling wife has been my own steadfast support throughout".

The 95-year-old monarch was marking the occasion -- which is also the anniversary of the death of her father, king George VI -- in private
The 95-year-old monarch was marking the occasion -- which is also the anniversary of the death of her father, king George VI -- in private POOL via AFP / Steve Parsons

The heir to the throne also paid tribute to the Queen's "devotion to the welfare of all her people", which "inspires still greater admiration with each passing year".

The Queen said she hoped that when Charles becomes king, the British people would give him and Camilla "the same support that you have given me".

Camilla was long vilified for her role in the break-up of Charles' marriage to Princess Diana.

Recognising the sensitivities, when the couple married in 2005, the royal family announced she would be known as Princess Consort after Charles became king.

The queen has been shown cards and other items sent by members of the public to mark her Platinum Jubilee and previous landmarks in her reign
The queen has been shown cards and other items sent by members of the public to mark her Platinum Jubilee and previous landmarks in her reign POOL via AFP / Steve Parsons

But she has gradually won plaudits as the future king's loyal wife.

Queen Elizabeth II was crowned at a ceremony at Westminster Abbey in June 1953
Queen Elizabeth II was crowned at a ceremony at Westminster Abbey in June 1953 INTERCONTINENTALE via AFP

Londoners who spoke to AFP were divided over the announcement.

"I'm delighted. I think it's high time," said Angela Roberts, an 80-year-old retiree.

"It's before my time but a lot of the people who grew up with Diana being the next Queen effectively, they feel like she was wronged by the royal institution," said Tobias Fox, a 24-year-old software engineer.

"I don't think a lot of people will be too happy that Camilla has got this role."

Profile of UK's Camilla, the Queen Consort-in-waiting.
Profile of UK's Camilla, the Queen Consort-in-waiting. AFP / Gal ROMA

"I think she'll do well, but I think there will always be that shadow of the past that overcasts her reign," said Alice Tomlinson, a 25-year-old working in marketing.

Princess Diana died in a car crash in Paris in 1997.

Stressing that the Queen is still actively working, Buckingham Palace released a photo taken at Sandringham this week showing her going through one of her famous red dispatch boxes used for government business.

Behind her is a photo of her late father.

Prime Minister Boris Johnson praised the Queen's "inspirational sense of duty and unwavering dedication".

With the main Platinum Jubilee celebrations set for June, he said wanted "to come together as a country to celebrate her historic reign".

World leaders paid tribute Sunday including German Chancellor Olaf Schulz and Canada's Justin Trudeau, who said she "has been a constant presence in the life of Canadians, offering steadfast leadership in times of change, hardship, or uncertainty".

The White House on Twitter congratulated the monarch on the "historic" milestone.

"Throughout the past 70 years, she has strengthened the ties of friendship, shared ideals, and faith in democracy that forever unite our countries," it said.

Four days of festivities are planned for early June, coinciding with the anniversary of her 1953 coronation, including a military parade and music concert, street parties, a nationwide "Big Jubilee Lunch" and a "Platinum Pudding Competition".

During her reign, the Queen has remained a constant through periods of huge social and political upheaval -- a living link to Britain's post-war and imperial past.

In her message addressed to the public signed "Your servant, Elizabeth R", the Queen renewed a pledge she first gave in a broadcast on her 21st birthday "that my life will always be devoted to your service".

In September 2015, she surpassed Queen Victoria's 63 years and seven months on the throne and, despite some health concerns over the past year, her latest message showed she is determined to continue her record-breaking reign.

After husband Philip's death in April last year, the Queen returned to public and official engagements, including hosting world leaders at the G7 summit.

Since then, she has largely stayed at Windsor Castle and made few public appearances.

But on Saturday, the Queen held a reception for locals at Sandringham, reportedly her largest in-person public engagement since the autumn health scare.