Saudi Arabia's 90-year-old King Abdullah on Thursday appointed his half-brother Moqren, 69, as the next heir to the throne of the world's largest exporter of crude oil.`
The decision, announced in a royal decree, comes as a source close to the circle of power told AFP that current Crown Prince Salman, 79, was sick and "may decide not to claim the throne" because of his ill health.
The decree did not mention Prince Salman, who is also defence minister of the strategic Gulf state.
Under the rules of succession in Saudi Arabia, power passes from brother to brother under the right of primogeniture among the sons of Abdul Aziz bin Saud, the kingdom's founder.
The king made public his decision on the eve of a visit by US President Barack Obama, who is expected in Riyadh late on Friday afternoon.
Under Thursday's decree, Prince Moqren, currently second deputy prime minister, is named as the next crown prince.
He will be proclaimed sovereign "if the posts of crown prince and king become vacant," according to the decision taken by "more than three-quarters" of the 34-member Board of Succession, the princes of the royal family.
The act is irrevocable, the decree said, stipulating that "nobody can change this decision," which takes effect from Thursday.
"King Abdullah wants to assure a smooth succession by this internal reorganisation of power" within the family, another source told AFP, adding that the decision had been "taken in agreement with Crown Prince Salman".
"It was passed by 27 of the 34 members. Others had reservations or abstained in the vote," the source said, speaking on condition of anonymity and without elaborating.
The Board of Succession is supposed to designate the future heir.
But a source close to royal circles in Riyadh told AFP King Abdullah had met the board and asked it to approve his decision to appoint Moqren crown prince "in case of the accession to the throne of Crown Prince Salman, or his absence."
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- Youngest of 35 sons -
The son of a Yemeni mother, and the father of 13 children including six sons, Prince Moqren is the youngest of the 35 sons of the country's founder.
He joined the air force in 1968 before becoming governor of Hael in the north and later the holy city of Medina in the west.
From 2006 until July 2012 he headed the intelligence services before being named counsellor to and special envoy to King Abdullah.
Diplomats say he is "one of the principal confidants" of the king.
In February last year, Prince Moqren was named second deputy prime minister.
The source said the king also informed the board of his intention to appoint his son Mitab as second deputy premier.
"About two-thirds of the board members approved" the appointment of Mitab, according to the same source.
He added that Prince Salman asked that support be given to his son, Prince Mohammed, to be appointed to the defence portfolio.
King Abdullah established the board in 2006 to institutionalise the process of transition, which would normally exercise its prerogatives after the monarch's death.
Analysts believe that the ageing Al-Saud dynasty should consider moving to the next generation within the ruling family for the succession.
But they also say that doing so could run the risk of igniting rivalries among the sons of dead kings or those of the present sovereign.