For the first time since 1973 the Queen will not be attending the Commonwealth heads of government meeting this year, Buckingham Palace has said.
She will be represented by Prince Charles at the summit in Sri Lanka in November.
Buckingham Palace said it was reviewing the amount of long-haul travel that is taken by the Queen.
The Queen, 87, is the head of the Commonwealth and every two years leaders meet to discuss global issues.
The Queen was first present at the Commonwealth heads of government meeting (CHOGM) in Ottawa, Canada, in 1973 - missing the first one in 1971 - and has been at every summit since. The last one, in 2011, was held in Perth, Australia.
'Transition'A Buckingham Palace spokesman said: "I can confirm the Queen will be represented by the Prince of Wales.
"The reason is that we are reviewing the amount of long-haul travel that is taken by the Queen."
This is a significant decision for the Queen and for her eldest son and heir.
The 87-year-old monarch won't have arrived at it lightly - she hasn't missed a heads of government meeting since 1973 and she regards the Commonwealth as an important dimension of her reign.
It is an acknowledgement of her advancing age and not, her officials insist, in order to avoid the political question about whether or not Sri Lanka should be the host in the first place.
The focus of campaigners, who are critical of Colombo's human rights record, will now be the Prince of Wales.
Behind the scenes, the ground has been prepared for him to become the organisation's next head - a position he's not automatically entitled to.
His attendance, in November, in place of his mother, will be another very visible sign of his preparation for kingship.
BBC royal correspondent Peter Hunt said the Queen would not have taken the decision lightly and it was both surprising and significant.
He said it was a significant moment for the Prince of Wales and it was also a symbolic move.
"It is about transition, about preparing this country for an elderly head of state who will be able to do less and less," he said.
"There is no intention of abdication. It will just not happen during her reign. It is the palace addressing the practicalities of her advancing age - you will see less of her and more of him."
By stepping in for the Queen, Prince Charles will be performing one of his most significant duties to date as a future King.
He has never before attended in place of the monarch at the two-yearly gathering of Commonwealth leaders. In 2007, both the Prince and the Queen attended CHOGM in Uganda.
The Queen was forced to cancel her appearance in March at the Commonwealth Day service at Westminster Abbey, the first time the monarch had missed the occasion in 20 years.
It was one of a number of engagements she cancelled after being admitted to hospital suffering from the symptoms of gastroenteritis, which can cause vomiting, fever and stomach ache.
Politically trickyThe Queen sees the Commonwealth as a "family" and takes pride in its work.
The Queen is the Commonwealth's symbolic head and has no formal powers over the 54 countries and two billion citizens which make up the voluntary association.
Commonwealth Secretary General Kamalesh Sharma said the "family of nations completely understands and respects" the Queen's decision.
"The presence of the Prince of Wales at our Commonwealth gathering will carry forward Her Majesty's outstanding record of enduring commitment and diligent service as head of the Commonwealth," he said.
Concerns have been raised about the choice of Sri Lanka as the host nation for the 2013 summit.
Campaigners including Amnesty International are calling for the CHOGM meeting not to take place there before an investigation is carried out into human rights abuses in the final six months of the 26-year Sri Lankan civil war.
Britain is facing pressure to lead a boycott of the meeting, with Canada's government indicating it will not attend unless specific criteria are met.
But Buckingham Palace said the Queen's decision was not related to the political situation.
"The key point here is that the Queen will be represented, although she is not there in person, by the Prince of Wales," a spokesman said.