Saudi Arabia's Crown Prince Nayef bin Abdulaziz al-Saud, next in line to rule the world's top oil exporter, has died just eight months after becoming heir to 89-year-old King Abdullah, the royal court said on Saturday.
Analysts and former diplomats said the succession process was likely to be stable, however, with the king and a family council expected to start work on the appointment of a new crown prince, who would probably be another brother of King Abdullah.
"With deep sorrow and grief... King Abdullah mourns his brother... Crown Prince Nayef who passed to the mercy of God on Saturday outside the kingdom," said a royal court statement carried by state media.
State TV said Nayef had died in Geneva where he had been receiving medical treatment for an unknown problem - he was thought to be 78. His death was not expected to trigger any major changes to the kingdom's energy policy or to key relationships with the United States and other allies.
"The fundamental principle that the Saudis operate under is stability. So they will I'm sure develop a consensus among the senior members of the family over an orderly succession. That has likely been forming in recent months in any event," said Robert Jordan, U.S. ambassador to Riyadh from 2001-03.
Defence Minister Prince Salman, 76, has long been viewed as the next most senior prince after the late Nayef. If he became king, analysts believe he would continue King Abdullah's cautious reforms.
Nayef, interior minister since 1975, was appointed crown prince in October after the death of his elder brother and the previous heir Crown Prince Sultan.
State television said the burial would be in Mecca on Sunday.
In a statement, British Foreign Secretary William Hague expressed his government's condolences, saying he was sad to hear of Nayef's death. "He served the Kingdom for many years with great dignity and dedication and his contribution to the prosperity and security of the Kingdom will be long remembered," said Hague.
The king of neighbouring Bahrain ordered a three-day mourning period, Bahrain News Agency said.
Nayef had a reputation as a steely conservative who opposed King Abdullah's reforms and developed a formidable security infrastructure that crushed al Qaeda but also locked up some political activists.
"He supervised the security affairs of the state for more than 30 years. He scored a lot of successes there. Especially in fighting al Qaeda," said Khalid al-Dakhil, a Saudi political analyst.
In May, Nayef went to Switzerland for medical tests, his second trip abroad for check ups for an undisclosed health issue since March. Like his brothers King Abdullah and Salman, he was one of the nearly 40 sons of Saudi Arabia's founder, Abdulaziz ibn Saud, who established the kingdom in 1935.
Prince Salman, his likely successor, was made defence minister in November and had served as Riyadh governor for five decades.
While Salman has often met foreign diplomats and other officials, he is seen as something of an unknown quantity, having maintained strong relations with both conservative clerics and western-oriented businessmen.
King Abdullah in May hosted a summit for Gulf Arab leaders and has looked well, if tired, in recent television appearances, but in October had his third round of back surgery in 12 months.
In a U.S. embassy cable released by WikiLeaks, American diplomats in 2009 reported that although Abdullah was in good health he "works only a few hours every day".
The conservative Islamic kingdom emerged from last year's Arab uprisings as one of the most stable Middle Eastern states.
Although most Saudi watchers say it is very likely that Salman will become the kingdom's leader after the deaths of Abdullah and Nayef, they say it is uncertain who would then be seen as next in line.
Although nearly 20 of King Abdulaziz's sons still survive, few of them have the requisite government experience to lead the country.
Meanwhile, those that have served for a long time in important positions, such as Deputy Interior Minister Prince Ahmed or Intelligence Minister Prince Muqrin, are younger than the oldest of King Abdulaziz's grandsons.
Under Saudi law, the line of sons must be exhausted before moving on a generation. But it might be seen as embarrassing for elder grandsons, who come first in the official line of precedence, to be overruled by their younger uncles.
Mecca Governor Prince Khaled al-Faisal, a son of the late King Faisal and brother to Foreign Minister Prince Saud al-Faisal, is seen as one possible contender among the next generation.
Another is Deputy Interior Minister Prince Mohammed bin Nayef and Eastern Province Governor Prince Mohammed bin Fahd.
Under succession rules drawn up six years ago, a new king has to nominate his choice of crown prince for approval by a family "allegiance" council. Although the council was involved in the appointment of Nayef as crown prince in October, it is not clear whether it voted on Abdullah's choice or was simply informed of it.