(reuters) -- 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 [Persian] 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.

An Arab source with close Saudi links also provided a similar account of the meeting at which King Salman asked MbN to step down because of his alleged drug addiction.

These sources said MbN had shrapnel in his body that could not be removed and he depended on drugs such as morphine to alleviate the pain. One source said MbN had been treated in clinics in Switzerland on three occasions in recent years. Reuters was unable to confirm this independently.

A Palace Coup

The King moved ahead of a meeting of the Political and Security Council. The meeting was due to start at 11 pm, but a few hours before that, MbN received what he viewed as a routine phone call from Mohammed bin Salman. According to the source close to MbN, Mohammed bin Salman told MbN that the king wanted to see him.

In the hours that followed the meeting in which MbN was dismissed, the House of Saud's Allegiance Council, comprising the ruling family's senior members, were informed of a letter written in the name of the king.

Drafted by palace advisers to MbS, it said MbN had a medical condition - drug addiction - and "we have been trying for over two years to persuade him to seek treatment but to no avail".

"Because of this dangerous situation we see that he should be relieved of his position and that Mohammed bin Salman be appointed in his place," the Saudi source close to MbN quoted excerpts of the letter as saying.

The letter was read over the phone to members of the Allegiance Council, while MbN was kept isolated in a room all night, his mobile phone removed, and cut off from contact with his aides. His bodyguards from elite paramilitary interior ministry units were also replaced.

Envoys were sent to council members to get their signatures. All but three of 34 signed. The coup had worked.

Calls by council members who backed MbN's removal were recorded and played to him by a palace adviser to demonstrate the strength of the forces against him and to discourage any urge the 57-year-old crown prince might have to resist.

According to two Saudi sources with links to the royal house, only three members of the council opposed his overthrow: Ahmed bin Abdulaziz, a former interior minister, Abdulaziz bin Abdallah, a representative of the family of late king Abdallah, and Prince Mohammad bin Saad, a former deputy governor of Riyadh. The three could not immediately be reached for comment.

At dawn MbN gave up. He told a palace adviser that he was ready to see the king. The meeting was short. MbN agreed to step down and signed a document to that effect.

When MbN left the king's quarters, he was surprised to see MbS waiting for him, the adviser said. MbN was embraced and kissed by MbS while television cameras rolled.

Soon afterward a pre-written statement was released announcing the king's decision to make his son the next crown prince. This was the clip that would play on all Saudi and [Persian] Gulf media over the coming hours and days.

House Arrest

MbN remains under house arrest to keep him out of circulation following his overthrow, with no visitors allowed except close family members. He is not taking calls, the source close to MbN said. In the past week he was only granted permission to visit his elderly mother with the new guards assigned to him.

The senior Saudi official said, however, that MbN had received guests, including the king and the new crown prince.

The source close to MbN said he would like to take his family to Switzerland or London but the king and MbS had decided that he must stay. "He wasn't given any choice."

The White House and CIA declined to comment. A senior administration official said Washington knew that MbS was the favourite of the king but "beyond that it's very opaque".

The elevation of MbS had been predicted by some Saudi and Western officials, but it came much sooner than expected with a rushed exit for MbN.

Since King Salman's accession, there had been clear indications that MbS was favoured over MbN, setting the stage for the younger prince to eclipse the formal heir to the throne.

MbS was given unprecedented power by his ailing 81-year-old father, which he used to reorder the top jobs in the political, oil, security, security and intelligence sectors, often without the knowledge of MbN, according to diplomats and Saudi political and security sources.

Since Salman took the helm just over two years ago, MbS has placed his men in key positions. MbS has been interfering in MbN's interior ministry, appointing, promoting and firing officers without informing him.

The succession quarrel, the sources said, began in 2015 when MbN's personal court was disbanded and merged with the court of the king, preventing MbN from bestowing independent patronage and cultivating support. This was followed by the sacking of Saad al-Jabri, MbN's security adviser.

When Donald Trump entered the White House, MbS cultivated contacts in Washington to offset the strong support that MbN had in the U.S. security and intelligence establishment because of his successes against al Qaeda.

The source close to MbN told Reuters the putsch went ahead after MbS struck up a strong relationship with Trump's son-in-law and adviser, Jared Kushner.

A White House official declined comment when asked about Kushner’s relationship with MbS.

The official, referring to MbN's removal as Crown Prince and MbS’ ascension to the post, said:

“The United States government also sought not to intervene or to be seen as intervening in such a sensitive internal matter. We have great respect for the King, Prince Mohammed bin Nayef and Prince Mohammed bin Salman and we consistently stressed our desire to maintain cooperation with the KSA (Kingdom of Saudi Arabia) and its leadership. This message was communicated at all levels of government.”

With MbS's sudden ascent, there is now speculation among diplomats and Saudi and Arab officials that King Salman is poised to abdicate in favour of his son.

Quoting a witness at the palace, one Saudi source said King Salman this month pre-recorded a statement in which he announces the transfer of the throne to his son. The announcement could be broadcast at any time, perhaps as soon as September.