The Duchess of Cornwall has decided against attending a memorial service for the 10th anniversary of the death of Princess Diana, Clarence House said.
Princess Diana was killed in a car crash in 1997
Camilla had accepted her invitation by Princes William and Harry to the event.
But she has now pulled out of Friday's service, fearing her attendance "could divert attention from the purpose of the occasion".
The hour-long service will be held by Buckingham Palace, in the Guards' Chapel in London's Wellington Barracks.
The Duchess is said to have talked at length with Prince Charles and her family about whether she should attend the thanksgiving service.
In a statement explaining her decision, Camilla said she was "touched" at her invitation from Princes William and Harry.
"I accepted and wanted to support them," she said.
"However, on reflection I believe my attendance could divert attention from the purpose of the occasion which is to focus on the life and service of Diana."
"I'm grateful to my husband, William and Harry for supporting my decision." An aide said: "It was never going to be an easy decision either way."
Buckingham Palace said that the Prince of Wales and the Duchess would have consulted the Queen over their decision.
"The Queen is certainly aware. The Queen would ultimately have supported the decision," a Palace spokeswoman said.
The BBC's royal correspondent Nicholas Witchell said he believes there will be "relief" at Buckingham Palace over Camilla's decision.
He says the Queen and her senior advisers are believed to have had serious misgivings about her presence at the service.
Our correspondent says this is because it would have gone against one of the key lessons the royal family was supposed to have learnt after Diana's death, which was that it "cannot act in a way that flies in the face of public feeling and the public mood".
Dickie Arbiter, former press secretary to the Queen, told the BBC the Duchess of Cornwall should have acted earlier.
Camilla said her presence could 'divert attention' from the event
"It could have quite simply been done right at the very beginning when the idea was first mooted to have a service, with a short statement saying the Duchess of Cornwall would love to support her husband, would love to support William and Harry, but she feels that it was inappropriate to attend.
"That would have been the story of the day. It would have been done and dusted ... without all this shilly-shallying, all the way up to the memorial service, which is literally five days away."
Around 500 guests are expected at the thanksgiving service, including more than 30 royals and celebrities such as Sir Elton John, and Prime Minister Gordon Brown.
Princes William and Harry are to give readings along with Diana's sister, Lady Sarah McCorquodale.
Princess Diana died, aged 36 - along with her companion Dodi Al Fayed, 42, and chauffeur Henri Paul - when the Mercedes they were in crashed in the Pont de l'Alma tunnel in Paris on 31 August 1997.