William and Harry, who were just 15 and 12 respectively when their mother died following a high-speed car crash in Paris, have spent months arranging the service at the Guards Chapel, near Buckingham Palace in central London.
But the event could be overshadowed by a row over the guest-list - after Camilla, Duchess of Cornwall, the second wife of Prince Charles, and the woman Diana described as the third person in her marriage, pulled out Sunday.
Some 500 high-profile guests will attend the memorial service, including Queen Elizabeth II, Charles, Prime Minister Gordon Brown, and his predecessor Tony Blair.
Diana's brother Earl Charles Spencer, whose funeral address criticized the royal family, and two sisters Lady Sarah McCorquodale and Lady Jane Fellowes, will also be present, as will Sir Elton John, who sang his song "Candle In The Wind" at the 1997 service.
William and Harry, now officers in the British army, will give readings, while Archbishop of Canterbury Rowan Williams, head of the Anglican church, has written two special prayers for the service.
Camilla, though, will be conspicuous by her absence - opting to spend the day at her private home in Wiltshire, south England, after a U-turn when she said she did not want to "divert attention from the purpose of the occasion."
Also not attending is Mohamed Al Fayed, the father of Diana's lover Dodi Fayed, who also died in the Paris car crash.
Fayed will instead hold a two-minute silence in the upmarket Harrods department store in London, which he owns, and where he has erected a statue of Diana and Dodi holding hands and releasing a bird.
Fayed claims that Diana and his son were killed in an establishment plot to stop the princess from marrying a Muslim.
But last year, a report from Lord John Stevens, former head of London's Metropolitan Police, ruled out any conspiracy, and said that the crash, which also killed the car's driver Henri Paul, was a "tragic accident."
French investigators concluded that Paul, an employee of the Fayed-owned Ritz Hotel, was well over the legal alcohol limit when he drove Diana and Dodi toward the latter's Paris apartment with paparazzi in hot pursuit.
Diana's death generated an unprecedented outpouring of public grief in Britain.
One million people took to the streets of London for the funeral, and some say the episode changed the country.
The tragedy also forced the royal family to present itself as a more open and inclusive institution.
Diana fans have already pinned flowers and poems to the gates of her former home, Kensington Palace in London, where a carpet of flowers built up after her death, and are expected to hold their own mini-service later in the day.
A YouGov poll published in The Daily Telegraph Friday suggested that "respect" for the royal family had dipped to 49 percent - a historic low.
Journalist Julie Burchill, who has written a biography of Diana, wrote in a special supplement in Britain's biggest-selling newspaper, the Sun, Friday that "she was one of us."
"What would make the Windsors hate and fear her as a 'loose cannon' was exactly the thing that made her so beloved to the rest of us - her big, beating, messy, caring heart," she wrote.
Kensington Palace is also hosting an exhibition in her memory, as is London's National Portrait Gallery.
Diana's childhood home, Althorp, in central England, where she is buried, is breaking with tradition by opening to the public on the anniversary.
And the British Broadcasting Corporatation is rescreening her funeral in full on a digital channel.
© 2007 Agence France-Presse
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