This was contained in a royal decree published by the official Saudi Press Agency.

Prince Mohammed, 37,  is already de facto ruler of the kingdom and the appointment formalizes his role as leader of the government.

The crown prince, known by his initials MBS, previously served as deputy prime minister as well as defence minister.

He is being replaced as defence minister by his younger brother, Khalid bin Salman, who was deputy defence minister.

Meanwhile, President Muhammadu Buhari has congratulated Prince Salman  on his new appointment.

In a letter he personally signed, the president welcomed the appointment which is combined with the position of defence minister, the first ever in the history of the Kingdom.

Buhari, in a statement issued by his media aide, Garba Shehu, said the government and the people of Nigeria looked forward to working with him to further deepen and expand excellent bilateral relations.

The Nigerian leader expressed confidence that with the dynamic and visionary leadership of the government by the Prime Minister under the guidance of his father, ruler of the Kingdom and Custodian of the two Holy Mosques, King Salman Bin Abdulaziz, the comprehensive strategic partnership of the two states will continue to blossom.

President Buhari however wished Salman a successful tenure in office and prayed to Allah to increase his health and wisdom.