Saudi King Abdullah appointed one of his brothers, Moqren bin Abdul Aziz, as deputy prime minister, on Friday, making him second in line to the throne, the official SPA news agency said.
"Prince Moqren bin Abdul Aziz, adviser and special envoy of the king, has been named second vice president of the Council of Ministers," said the SPA report.
The king, 89, is the president of the Council, and Crown Prince Salman bin Abdul Aziz, 77, is the first vice president.
In the event of the king's death, the 68-year-old Moqren could then become crown prince.
Like Prince Salman, Moqren is a son of Abdul Aziz bin Saud, who founded Saudi Arabia and died in 1953.
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He has served as an adviser and special envoy of King Abdullah since July, previously serving as the head of Saudi intelligence.
The monarch's age and frequent hospitalisation have raised concerns about the future leadership of the ultra-conservative Gulf kingdom, a key player in the Middle East and a major exporter of oil.
The post of deputy premier had been vacant since Prince Nayef bin Abdul Aziz, a half-brother of the king, became crown prince in October 2011 following the death of Prince Sultan and then died himself last June.
Under the rules of succession in Saudi Arabia, power passes from brother to brother, respecting the law of birthright among the sons of the kingdom's founder.
So far, five sons of the late king Saud have ruled since the kingdom's founder died. Eighteen of Saud's sons are still alive, although experts say few could be considered future monarchs.
In addition to Prince Moqren, the most high profile are former interior minister Prince Ahmad and the governor of Riyadh, Prince Sattam.
Analysts believe the aging Al-Saud dynasty will have to contemplate passing the baton to a new generation, the grandsons of the kingdom's founder.