Reuters - On Tuesday June 20 Mohammed bin Nayef, a powerful figure in Saudi Arabia's security apparatus for the past two decades and the next in line to the throne, was summoned to meet King Salman bin Abdulaziz on the fourth floor of the royal palace in Mecca.


There, according to a source close to MbN, as he is known, the king ordered him to step aside in favour of the king's favourite son, Mohammed bin Salman. The reason: an addiction to painkilling drugs was clouding MbN's judgment.


"The king came to meet MbN and they were alone in the room. He told him: 'I want you to step down, you didn't listen to the advice to get treatment for your addiction which dangerously affects your decisions'," said the source close to MbN.


The new details about the extraordinary meeting between the king and MbN that touched off the de facto palace coup help to explain the events that are reshaping the leadership of the world's biggest oil exporting nation.


Reuters could not independently confirm MbN's addiction issues.


A senior Saudi official said the account was totally "unfounded and untrue in addition to being nonsense".


"The story depicted here is a complete fantasy worthy of Hollywood," the official said in a statement to Reuters, which did not refer to MbN's alleged use of drugs.


The official said MbN had been removed in the national interest and had not experienced any "pressure or disrespect". Reasons for his dismissal were "confidential".


Sources with knowledge of the situation said however that the king was determined to elevate his son to be heir to the throne and used MbN's drug problem as a pretext to push him aside.


Three royal insiders, four Arab officials with links to the ruling house of Saud, and diplomats in the region, told Reuters that MbN was surprised to be ordered to step aside.


"It was a big shock to MbN," said a Saudi political source close to MbN. "It was a coup. He wasn't prepared."


The sources said MbN did not expect to be usurped by the often impulsive Mohammed bin Salman, who MbN considered to have made a number of policy blunders, such as his handling of the Yemen conflict and cutting financial benefits to civil servants.


The high-stakes power grab has placed sweeping powers in the hands of the 32-year-old Mohammed bin Salman, also known as MbS, and appears designed to speed his accession to the throne.


Should he get the job, the young prince will preside over a kingdom facing tough times from depressed oil prices, the conflict in Yemen, rivalry with an emboldened Iran and a major diplomatic crisis in the Gulf.


The source close to MbN acknowledged that he had health issues, which were aggravated after an al Qaeda attacker tried to blow himself up in front of him in his palace in 2009. The health issues were corroborated by three other sources in Saudi Arabia and Arab official sources with links to the royal family.