Saudi Arabia's Crown Prince Sultan bin Abdulaziz al-Saud died this morning of colon cancer in a New York hospital.
Sultan, whose official age was given as 80, had been heir to the throne of the world's top oil exporter since 2005 but was a central figure in Saudi decision making since becoming defence minister in 1962.
"With deep sorrow and sadness the Custodian of the Two Holy Mosques King Abdullah bin Abdulaziz mourns the death of his brother and his Crown Prince Sultan... who died at dawn this morning Saturday outside the kingdom following an illness," said the statement carried on state news agency SPA and state television.
Sultan's health had declined in recent years and he spent long periods outside the kingdom for medical treatment. A 2009 US diplomatic cable released by WikiLeaks described him as "to all intents and purposes incapacitated".
King Abdullah is now expected to call to session an untested Allegiance Council to appoint his new heir, widely expected to be Prince Nayef, the veteran interior minister.
Set up in 2006 soon after Abdullah became king, the Allegiance Council is tasked with voting to approve the monarch's choice of crown prince or nominating its own choice instead.
The Allegiance Council consists of a representative from each of the 34 branches of the al-Saud ruling family who were born to Saudi Arabia's founder, King Abdulaziz Ibn Saud. Because some members are thought to be travelling, the council might not be able to convene to approve a new crown prince for some days. Previously, the succession was decided only by the king and a small coterie of top princes.
Unlike in European monarchies, the line of succession does not move directly from father to eldest son, but has moved down a line of brothers born to the kingdom's founder Ibn Saud, who died in 1953.
Prince Nayef has been interior minister since 1975 and was appointed second deputy prime minister in 2009, a position usually given to the man considered third in line to rule.
Thought to be more conservative than either King Abdullah or Prince Sultan, Prince Nayef is in his late 70s and has managed the kingdom's day-to-day affairs during absences of both the king and crown prince.
Saudi television broke its schedules early today to broadcast Koranic verses accompanied by footage of the Kaaba in Mecca, Islam's holiest site. Funeral services will be held in the Saudi capital Riyadh on Tuesday, SPA said.
US president Barack Obama expressed his "great regret" at the death of the prince and noted the important role he had played in cementing ties between the two countries. "He was a strong supporter of the deep and enduring partnership between our two countries forged almost seven decades ago," Mr Obama said in a statement, adding that he had been "a valued friend of the United States".
US secretary of state Hillary Clinton expressed her condolences on the death and said US ties with Saudi Arabia were "strong and enduring."
Meanwhile, Abdullah has left hospital after back surgery earlier this month and will continue medical treatment at a royal clinic, the Royal Court said in a statement tonight. The king was absent for three months late in 2010 while he underwent treatment for a herniated disc that caused blood to accumulate around his spine.
Reuters