Saudi Arabia's King could have his Crown Prince son replaced to restore the credibility of the monarchy amid turmoil over the Jamal Khashoggi murder, Britain's former defence attaché to the kingdom has claimed.

Crown Prince Mohammed bin Salman has faced claims he may have been behind the death of the journalist who went missing after entering the Saudi consulate in Istanbul.

Saudi Arabia first denied any role in Khashoggi's disappearance before blaming his October 2 death on a botched attempt to return him to the kingdom.

On Thursday, Jamal Khashoggi's eldest son left Saudi Arabia for Washington DC with his family after a travel ban intended to lure the journalist home was lifted.   

It came as Saudi prosecutors said the murder was planned and suspects were being interrogated.