Saudi Arabia's Crown Prince Nayef bin Abdul-Aziz al-Saud has died just eight months after becoming heir to 89-year-old King Abdullah.
State TV said on Saturday that Nayef had died in Geneva, where he had been receiving medical treatment for an unknown illness.
The 78-year-old, who also headed the interior ministry, was known as a reactionary whose views on issues such as women driving differed from those of the king and his death could mean more rapid reform.
His death underlines the age of the senior office-holders in the royal family, all of whom are sons of Abdul-Aziz Ibn Saud, who founded the desert kingdom in 1932. The king and a family council are now expected to nominate a new crown prince.
The defence minister, Prince Salman, 76, has long been regarded as the next most senior prince after Nayef. He is seen as a supporter of King Abdullah's cautious reforms, which have established some elections at a municipal level and encouraged mixed education for postgraduates, a radical shift in the fiercely conservative country.
However, Nayef remained hugely influential and enjoyed a lot of support among the powerful clerical establishment. He oversaw the feared religious police and developed a security apparatus that contributed to the failure of militant Islamic groups such as al-Qaida to capitalise on early successes in Saudi Arabia after the 2001 war in Afghanistan.
"He supervised the security affairs of the state for more than 30 years. He scored a lot of successes there. Especially in fighting al-Qaida," said Khalid al-Dakhil, a Saudi political analyst.
But Nayef was criticised for imprisoning human rights activists and political campaigners.
The Islamic kingdom largely escaped the instability of last year's Arab uprisings despite a repressive political system. Signs of unrest among the Shia Muslim minority were quickly stifled.
As revolts rocked the kingdom's neighbours, King Abdullah ordered a package of pay rises, subsidies, housing grants and job creation schemes. Nevertheless, social and economic problems remain, including finding jobs for many young people who often lack good quality education.
Abdullah, who has pushed planners to prepare for a time when the state can no longer rely solely on oil for revenue, enjoys a degree of popularity that few senior Saudi royals can match.