Controversial Saudi Crown Prince Mohammed bin Salman has been promoted to be the country's Prime Minister by his father King Salman.

Saudi Arabia's Crown Prince, known as MBS, has become the kingdom's prime minister, a royal decree said on Tuesday.  

Saudi King Salman ordered the cabinet reshuffle, giving his son the post which has traditionally held by the king.

MBS, who has been the kingdom's de facto ruler for several years, was previously serving as deputy prime minister under King Salman as well as defence minister. 

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

Prince Abdulaziz bin Salman remained Saudi Arabia's energy minister under a new cabinet headed by the kingdom's crown prince and de facto ruler Prince Mohammed bin Salman. 

The foreign minister Prince Faisal bin Farhan Al Saud, the finance minister Mohammed al-Jadaanand and the investment minister Khalid al-Falih kept their positions in the new cabinet. 

Prince Mohammed, who turned 37 last month, has been first in line to succeed his father as king since 2017.

He became defence minister in 2015, a key step in a swift consolidation of power that saw him take the lead on major portfolios.

Saudi Arabia has for years sought to quell speculation over the health of 86-year-old King Salman, who has ruled the world's top oil exporter since 2015.

In 2017, it dismissed reports and mounting speculation that the king was planning to abdicate in favour of Prince Mohammed.