The Queen celebrated 60 years on the throne on Monday to become the second British monarch to reach that milestone.
During her reign, the royal family has faced tumultuous times and moments of plummeting popularity, above all after the death in 1997 of Princess Diana, the hugely popular ex-wife of her son and heir-to-the-throne Prince Charles.
But the royals are enjoying fresh support after the wedding last year of the queen's grandson, Prince William, second-in-line to the throne, to Catherine Middleton.
Elizabeth II, 85, became queen aged 25 on February 6, 1952 on the death of her father George VI, while on tour in Kenya with her husband Prince Philip.
In a documentary for BBC TV to be shown on Monday, grandson Prince Harry, younger son of Charles, said how much she relied on Prince Philip. I don't think that she could do it without him, especially when they're both at this age, he said.
Ninety-year-old Prince Philip, the longest-serving consort and oldest-serving spouse of any British monarch, had surgery at Christmas to clear a blocked heart artery.
SCRUTINY AND CRITICISM
Accusations of excess and the divorces of three of the queen's children have marred the image of the royal family, which during her reign has come under intense media scrutiny.
The queen was personally criticised for appearing unmoved and out of touch with public feeling over the death of Diana, a moment captured in the 2006 Oscar-winning film The Queen.
That event left many questioning the monarchy itself.
The queen said in a statement that she was grateful for the wonderful support and encouragement she had received over the years, and promised to dedicate myself anew to your service.
I hope also that this Jubilee year will be a time to give thanks for the great advances that have been made since 1952 and to look forward to the future, she said.
The queen been on the throne for longer than any other monarch except Queen Victoria, who reigned from 1837 to 1901.
During her reign there have been 12 British prime ministers, 12 U.S. presidents and six popes. She has visited 116 countries.
CELEBRATIONS IN JUNE
Celebrations on Monday will be low key. The queen will spend the day carrying out formal duties in Norfolk, eastern England, visiting a town hall and a junior school.
There will be gun salutes at the Tower of London and at Hyde Park, while two specially commissioned photographs of the queen and Prince Philip will be published on a website launched to mark the jubilee. (www.thediamondjubilee.org)
Major celebrations take place over four days in June, including a concert at Buckingham Palace and a pageant involving a flotilla of 1,000 ships on the River Thames.
You sometimes hear people talk as if the monarchy was simply a glittering ornament, a decoration on our national life, Prime Minister David Cameron said in a statement.
That misunderstands our constitution and it underestimates our queen. All my life, and for the lives of most people in this country, she has always been there for us.
(Editing by Alessandra Rizzo)