RIYADH — King Abdullah of Saudi Arabia appointed Thursday his half-brother Moqren bin Abdul Aziz as the next heir to the throne, paving the way for him to become a future king.

The move was announced by a royal decree broadcast by state television channel Al-Ikhbariya.

Under an irrevocable decree, Prince Moqren, currently second deputy president of the council of ministers, is named the next crown prince.

There was no mention in the decree of current Crown Prince Salman bin Abdul Aziz.

Prince Moqren 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 decision is irrevocable, the royal decree said, stipulating that “nobody can change this decision” which takes effect from Thursday.

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, 69, crown prince “in case of the accession to the throne of Crown Prince Salman, or his absence.”

The source said Prince Salman, who is also defense minister, is sick and “may decide not to claim the throne” for health reasons.

Under the rules of succession in Saudi Arabia, the world’s largest oil exporter, power passes from brother to brother under the right of primogeniture among the sons of King Abdul Aziz bin Saud, the kingdom’s founder.

King Abdullah established the board in 2006 to institutionalize the process of transition, which would normally exercise its prerogatives after his death.

Earlier Thursday, a former wife of King Abdullah appealed to US President Barack Obama for help in the case of four daughters she says are being held in a royal palace.

Alanoud AlFayez, 57, a Jordanian national who has lived in London since her divorce from the Saudi monarch in 2003, said her children needed to be “saved.”

Her comments come ahead of Obama’s visit to the kingdom on Friday, during which US lawmakers have urged him to address rights violations in Saudi Arabia.