The Queen will not attend this year's Commonwealth heads of government meeting (Chogm) in Sri Lanka, Buckingham Palace has said, in a significant announcement auguring future changes to accommodate her age.
The Prince of Wales will represent the 87-year-old monarch, who has not missed the biennial conference since 1973.
It will be the first time the future king has stepped into this role. The move will be interpreted as preparing the way for him to become the next head of the Commonwealth, even though he has no automatic entitlement to the position.
There had been calls by campaigners for the Queen to avoid the November summit in Colombo because of Sri Lanka's human rights record.
The fact she has decided her eldest son should go in her place indicates this is not a political decision but a practical one.
Palace officials are reviewing the amount of long-haul travel undertaken by the monarch and the Duke of Edinburgh, who will be 92 next month.
A spokesman said: "It is nothing to do with the political situation in Sri Lanka. The key point here is that the Queen will be represented, although she not there in person, by the Prince of Wales".
The decision will not have been taken lightly. Apart from highlighting the Queen's age and arousing concern for her health, it will be seen as a symbolic moment in Charles's preparation for kingship and his assuming more of his mother's duties.
The Queen's regards her role as head of the 54-nation voluntary association – she is Queen of 16 Commonwealth realms and their 137 million people – as one of the most important of her reign, although she has no formal powers.
On opening the Commonwealth Institute building in west London in 1962, she said: "I suppose that between us, my husband and I have seen more of the Commonwealth than almost any people alive."
She has visited all but two Commonwealth countries: Cameroon, which joined in 1995, and Rwanda, which joined in 2009.
Prince Philip has described her as the "Commonwealth psychotherapist" and she has built up unique relations with its leaders throughout her reign. As a young queen she grew up alongside leaders such as Zambia's Kenneth Kaunda, Tanzania's Julius Nyerere and Indira Gandhi of India.
Her role means her relationship with the Commonwealth and its leaders is different to that of any British government. Many use Chogm as an opportunity to privately confide in her.
She was first present at the 1973 Chogm in Ottawa, and has been at every summit since. However, she did not attend the inaugural meeting in Singapore in 1971. The then prime minister, Edward Heath, effectively prevented her from going because of member states' attitudes to the question of Britain selling arms to South Africa. She is understood to have wanted to go, and regretted not going, but was bound by Heath's advice.
In 2007, unusually, both Charles and the Queen attended Chogm in Uganda. Though he did not attend the opening ceremony, which was attended by the Queen, Charles supported her on the visit and his presence kindled debate on whether he would assume the Commonwealth mantle.
It will be up to the countries' heads of government to decide what they want to do with the symbolic post in the future. Opposition has been voiced to Charles inheriting the role.
But, in the last few months, the Australian prime minister, Julia Gillard, seemed to support such a move. She said: "For Australia's part, I am sure the Queen's successor as monarch will one day serve as head of Commonwealth with the same distinction as Her Majesty has done".
The Commonwealth secretary general, Kamalesh Sharma, has also spoken of how Charles's support for the Commonwealth has deepened its links to the crown.
Campaign groups including Amnesty International are calling for the Chogm meeting not to take place in Colombo before an investigation is carried out into human rights abuses in the final six months of the 26-year Sri Lankan civil war.
