Diamond Jubilee: Queen celebrating 60-year reign
BBC: The Queen has said she is dedicating herself "anew to your service", as she marks the 60th anniversary of her accession to the throne.
In a message, she also said she felt "deeply moved" by the support shown for her Diamond Jubilee.
It will be marked by several events, with the main celebrations in June.
The Queen, 85, usually spends Accession Day - the day her father, George VI, died in 1952 - privately but this year has two engagements in Norfolk.
This year's anniversary will be marked by visits to King's Lynn Town Hall and the nearby Dersingham Infant and Nursery School.
Crowds are expected to gather to greet the Queen, despite the cold weather.
Two official photographs were released, and a 41-gun salute will be held in Hyde Park, London, followed by a 62-gun salute at the Tower of London.
During her reign, Queen Elizabeth II has seen 11 UK prime ministers come and go, with David Cameron her 12th.
Mr Cameron praised the "magnificent service" given by the Queen and called her a "source of wisdom and continuity".
"With experience, dignity and quiet authority she has guided and united our nation and the Commonwealth over six varied decades," he said.
One former Prime Minister, John Major, reflected on the counsel of the Queen and the many private meetings he had with her during his premiership during an interview on BBC One's Andrew Marr Show on Sunday.
He said: "Most of the present cabinet weren't born when the Queen became monarch. So there's very little she hasn't seen, very little she doesn't understand. And anyone who doesn't listen to her view and consult her when necessary is missing a huge opportunity."
Meanwhile, Sir John has formally launched the Queen Elizabeth Diamond Jubilee Trust to help those in need across the Commonwealth.
He will chair the grant-making body, which will focus on areas such as fighting curable diseases and the promotion of education and culture.
A set of six 1st class definitive stamps, all of which feature official portraits of the Queen, are also being issued to mark the monarch's milestone.
Moya Greene, Royal Mail's chief executive, said: "The Queen's image is one of the most recognisable in the world and we are delighted to bring these iconic images together to mark the 60th anniversary of the Queen's accession."
The Diamond Jubilee Miniature Sheet marks the first occasion on which official images of the monarch, sourced from stamps, coins and banknotes, have been brought together for a stamp issue.
The Queen's 60 years as monarch are set to be marked by a series of regional, national and international events during 2012, culminating in a four-day long UK Bank Holiday weekend in June.
Events planned to take place during the summer include the Queen leading a flotilla of a thousand boats along the Thames and a chain of beacons being lit across the country.
The Queen and the Duke of Edinburgh also intend to travel as widely as possible across the UK.
Other members of the Royal Family will visit the 15 other countries where the Queen is head of state, as well as some other Commonwealth countries.
In her message to mark the anniversary, the Queen said: "I am writing to thank you for the wonderful support and encouragement that you have given to me and Prince Philip over these years and to tell you how deeply moved we have been to receive so many kind messages about the Diamond Jubilee.
"In this special year, as I dedicate myself anew to your service, I hope we will all be reminded of the power of togetherness and the convening strength of family, friendship and good neighbourliness, examples of which I have been fortunate to see throughout my reign and which my family and I look forward to seeing in many forms as we travel throughout the United Kingdom and the wider Commonwealth."
The moment a princess became a queen
It is exactly 60 years since Princess Elizabeth, on a trip to Kenya, became Queen. What were the events that followed, and how prepared was the 25-year-old?
It was February 1952 and Princess Elizabeth was enjoying a short break in Kenya with Prince Philip, her husband of five years. It was a brief respite from their royal duties.
They were standing in for George VI on a long-planned international tour that was to also take in Australia and New Zealand. The 56-year-old King, thousands of miles away at Sandringham, had been too ill to travel.
The pair were relaxing at a game-viewing lodge, at the now-famous Treetops Hotel just over 100 miles (165km) from Nairobi. Elizabeth had spent the day of 5 February taking cine films of elephants at a nearby watering hole before retiring with the Duke of Edinburgh to their cabin high up in the trees. They spent the night surrounded by wild animals.
And it was here on 6 February, that the princess became Queen, after George VI passed away in his sleep.
The British hunter Jim Corbett, who was also staying at Treetops at the time, later wrote the now famous lines in the visitors' log book:
"For the first time in the history of the world, a young girl climbed into a tree one day a Princess and after having what she described as her most thrilling experience she climbed down from the tree next day a Queen."
At the time, however, Elizabeth had no knowledge of the event that was to change her life.
Because their location was so remote, the news of the King' s death took some time to reach the royal party. It came first to a senior courtier, who passed it to the princess's private secretary, Martin Charteris, who then telephoned Prince Philip's aide.
Elizabeth had returned to Sagana Lodge, a farm some 20 miles (32km) away that had been given to her by the Kenyan government as a wedding present, when Philip eventually broke the news to her.
After what would undoubtedly have been an emotional walk in the grounds with her husband, the 25-year-old who had become Queen put aside her grief to write letters apologising for cancelling the rest of her tour. Arrangements were made for her to return home immediately.
From Nairobi, the BBC's Frank Gillard filed a report. "How tragic to think that even this morning, as she sat at breakfast, talking about her father, and proudly describing how bravely he'd stood up to his illness, how well he'd recovered - sitting there in her yellow bush shirt and brown slacks - even at that moment her father was lying dead and she had succeeded to his vast responsibilities."
Robert Lacey, author of A Brief Life of the Queen, says she would have been prepared for the news, even if her father's death from a coronary thrombosis was a shock.
"Her private secretary carried sealed envelopes containing a draft Accession Declaration. She was ready but it was a secret that was shared with few people."
It is said that she reacted stoically, and showed little immediate distress. "She was sitting erect, fully accepting her destiny," Martin Charteris is quoted as saying in Lacey's book. No-one saw any tears.
But royal historian and biographer Sarah Bradford is sure that she would have cried later, in private.
"Although she didn't show it in public, she absolutely adored him. They were very close. He was the one who brought her up in the ways of the monarch. She read state papers when she was still a princess. She saw heads of state in an informal way."
George VI had waved off his daughter and her husband at London Airport on 31 January. He had had a series of operations for cancer, and his health was failing.
Princess Elizabeth had been carrying out more and more of his engagements, such as greeting foreign dignitaries and riding in the King's place for the Trooping the Colour.
On a wet and cold February day, special black-bordered editions of the newspapers appeared on the streets of London, announcing George VI's death, at the age of 56.
The news of a royal death was very controlled in those days, says historian Hugo Vickers - who acted as a consultant for the film The King's Speech.
"They had a code at Sandringham which was 'Hyde Park Corner'. The private secretary in Sandringham rang Buckingham Palace and got the private secretary there. He would have said 'Hyde Park Corner' which meant that the King had died.
"That was his cue to go and see Queen Mary, and then Churchill to inform them of the news. All these people had to be told before it could be officially announced.
"He was a highly respected monarch, who'd had an awful reign. He had come to the throne unprepared, gone through the war, and then his health broke down. His death would have been much less sensational than it might be today, but the mood was more respectful - it wasn't a time for speculation."
Valerie Lowe, from Herefordshire, was 12 years old when George VI died. "We were in the art room at school when we saw the union jack being hoisted to half-mast.
"We started wondering who might have died. One imaginative child suggested that Princess Elizabeth might have been eaten by a lion on her safari.
"I don't think any of us guessed it was the King, in spite of knowing that he was in poor health. Then we were summoned to the school hall and given the news. We all felt very solemn, but also excited at the thought of the beautiful young Princess Elizabeth becoming Queen."
Princess Elizabeth formally proclaimed herself Queen and Head of the Commonwealth and Defender of the Faith on 8 February, at a 20-minute meeting at St James's Palace.
One hundred and fifty Lords of the Council, representatives from the Commonwealth, officials from the City of London and other dignitaries witnessed the Accession.
From her official proclamation Queen Elizabeth II read: "By the sudden death of my dear father I am called to assume the duties and responsibilities of sovereignty.
"My heart is too full for me to say more to you today than I shall always work, as my father did throughout his reign, to advance the happiness and prosperity of my peoples, spread as they are all the world over."
When George VI died, there were only around 1.5m television sets in a population of about 50m. Most people would have heard the news on the wireless, on the BBC's Home Service, or the popular Light Programme.
"The way the media covers royal events has completely changed. [Elizabeth's] father's funeral was, for her, a very private occasion," says Sarah Bradford.
George VI's body lay in state for three days in Westminster Hall. Some 300,000 people filed by to pay their respects, and he was buried in St George's Chapel, Windsor, on 15 February.
His funeral procession was televised, an event which helped to spark the first wave of mass television purchases.
But the funeral service was broadcast by sound only. At the time television was still seen as rather a vulgar medium by the establishment, and it would have been deemed to be an intrusion. The then Archbishop of Canterbury labelled it as "potentially one of the great dangers of the world".
The Queen's Coronation on 2 June 1953 (a day estimated by meteorologists to be the likeliest to produce sunshine that summer - although, of course, it rained) would be the first such event to be properly televised, and the first to be genuinely witnessed by the people.
Additional reporting: Melissa Hogenboom