Queen Elizabeth II will not attend the Commonwealth Heads of Government meeting later this year, Buckingham Palace said Tuesday.
The UK queen will be represented at the gathering in Sri Lanka in November by her son, Prince Charles.
The reason for the decision is that Buckingham Palace is reviewing the number of long-haul flights the queen takes, a royal source said.
The source, who declined to be named in keeping with royal protocol, said the decision was not made because of medical advice.
The queen, who's 87, was briefly admitted to a London hospital in March, where she was treated for symptoms of gastroenteritis.
Prince Charles, who is the heir to the throne, has accompanied the queen to Commonwealth events in the past, the source said.
The queen is the head of the Commonwealth, a voluntary association of 54 countries.
Commonwealth Secretary-General Kamalesh Sharma said in a statement Tuesday the group of nations "completely understands and respects" the queen's decision not to attend.
He paid tribute to "the continuing dedication and deep sense of duty" the queen brings to her role as head of the Commonwealth.
He also welcomed her decision to send Charles to the Sri Lankan capital, Colombo, in her stead, saying there had been "close links" between the prince and the Commonwealth over many years.
Prince Charles previously attended a Commonwealth Heads of Government meeting in Kampala, Uganda, in 2007, he added.
Commonwealth leaders gather every two years to discuss Commonwealth and global issues, with the last meeting held in Perth, Australia, in 2011.
The leaders represent about a quarter of the world's countries and a third of the world's population, according to the Commonwealth secretary-general.
They work together toward shared goals in democracy and development, according to the Commonwealth website. After a formal opening ceremony and initial session, the leaders meet in a private setting, it says.
"The atmosphere is informal, encouraging a full and frank exchange of views, and decisions are reached by consensus," it says.