The queen has long been a major supporter of the 54-nation Commonwealth, and her decision to send Prince Charles to the November meeting in Sri Lanka is seen as a reflection of her 87 years and of efforts to reduce long-distance journeys.
She was briefly hospitalized for a stomach illness earlier this year, and did not attend the Commonwealth Day Observance service at Westminster Abbey on March 11. She has rarely taken time off for illness, having carried out more than 400 official engagements in 2012 — ranging from private meetings with the prime minister to ceremonial gatherings.
The summit brings together dozens of presidents and prime ministers from Britain's former colonies. The group includes countries from five continents and espouses an impressive set of values: Democracy, human rights, free trade, racial equality, the rule of law and world peace.
The queen missed the meeting in Singapore in 1971, amid a controversy over Britain's proposal to sell arms to South Africa.
The Sri Lanka session has also been controversial, and human rights activists have been pushing for a change of venue. New-York based Human Rights Watch has argued for the session to move unless "the government makes prompt, measurable and meaningful progress on human rights."
Rights groups and foreign governments have called for an international probe on the alleged war-time abuses in Sri Lanka's long civil war, which ended in 2009.
However, the queen's decision is not being billed as an effort to make a political statement.
Charles attended the Uganda summit with the queen in 2007, but attending in her place is seen as a big step for the prince of Wales.