Moving Nayef to the top of the succession ladder is not likely to pose any risks to Saudi Arabia's pro-Western policies and, in particular, its close alliance with Washington. But Nayef is a more mercurial figure than Saudi Arabia's current leader, King Abdullah, who has nudged ahead with changes such as promising women voting rights in 2015 despite rumblings from the country's powerful religious establishment.
Nayef has earned U.S. praise for unleashing internal security forces against suspected Islamic extremist cells in Saudi Arabia, home to 15 of 19 of the Sept. 11 hijackers. Yet he brought blistering rebukes in the West for a 2002 interview that quoted him as saying that "Zionists" benefited from the 9/11 attacks because they turned world opinion against Islam and Arabs.
Nayef also has expressed displeasure at some of Abdullah's moves for openness, saying in 2009 that he saw no need for women to vote or participate in politics. His view is shared by many Saudi clerics, who follow the strict brand of Islam known as Wahhabism. Their support gives the Saudi monarchy the legitimacy to rule over a nation holding Islam's holiest sites.
"Nayef is more religious, and is closer to the Saudi groups who are very critical of the king's decisions regarding women and other steps he's taken to balance out the rigid religious practices in society," said Ali Fakhro, a political analyst and commentator in Bahrain.
It is doubtful that Nayef, if he became king, would annul Abdullah's changes, which include the establishment of a coed university. More likely, he would put any further changes on hold, said Abdulkhaleq Abdulla, a political affairs professor at Emirates University.
"It's not good news for Saudis or for the region," he said. Nayef "is the security guy. He is the mukhabarat [secret police] guy. He is the internal affairs guy."
After Sultan fell ill two years ago, Nayef was named second deputy prime minister, traditionally the post behind the crown prince. For the first time, however, the mechanism of picking the next No. 2 in the royal succession is not entirely clear.
Traditionally, the king names his successor. But Abdullah may put the decision to the Allegiance Council, a 33-member body composed of his brothers and cousins. Abdullah created the council and gave it a mandate to choose the heir for the first time when Sultan rose to the throne. But it was not specified whether it would be used if Sultan died before the king.
The choice of whether to convene the council will likely be made by Abdullah, 87, who is recovering from his third operation in less than a year to treat back problems.
"It is with deep sorrow and grief that the custodian of the two holy mosques, King Abdullah, mourns the loss of his brother and crown prince, His Royal Highness Prince Sultan," the palace said in a statement.
According to a leaked U.S. diplomatic cable from January 2010, Sultan had been receiving treatment for colon cancer since 2009.