Camilla, Duchess of Cornwall, had accepted an invitation from Charles and his sons, Princes William and Harry, to accompany them to the service Friday at the Guards' Chapel in London's Wellington Barracks. Some criticized the decision, since Camilla had an affair with Charles when he was still married to Diana.
"I'm very touched to have been invited by Prince William and Prince Harry to attend the thanksgiving service for their mother Diana, Princess of Wales," the duchess said in a statement. "I accepted and wanted to support them. 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."
Buckingham Palace said Queen Elizabeth II also supported this decision.
"I couldn't be happier if I'd won the lottery," said Joan Berry, secretary of the Diana Appreciation Society.
Diana, known for her charity work and tabloid celebrity, died in a Paris car crash with her boyfriend, Dodi Fayed, on Aug. 31, 1997.
Diana married Charles in 1981 in a ceremony at St. Paul's Cathedral televised around the world. They had two sons, William and Harry, but divorced in 1996 after admissions of adultery on both sides.
Friday's anniversary will be marked with a nationally televised memorial service and specially commissioned prayers.