Chinese has put feared ex-security chief, Zhou Yongkang, under formal arrest to investigate his suspected crimes, including accepting bribes, adultery and leaking the country's secrets, the official Xinhua News Agency said.

Zhou, a former member of the powerful nine-member Politburo Standing Committee, was also expelled from the Communist Party, making him the most senior figure to be snared in President Xi Jinping's anti-corruption crackdown.

What Zhou did completely deviated from the Party's nature and mission, and seriously violated Party discipline. His behaviors badly undermined the reputation of the Party, significantly damaged the cause of the Party and the

people, and have yielded serious consequences. Xinhua News Agency report

The square-jawed, granite-faced Zhou is the highest-level official to be prosecuted since the 1981 treason trial of Mao Zedong's wife and other members of the "Gang of Four" who persecuted political opponents during the 1966-76 Cultural Revolution.

He had been under investigation for "severe disciplinary violations" - a phrase usually used to describe corruption - since July. He had not been seen publicly since October 2013.

"He abused his power to help relatives, mistresses and friends make huge profits from operating businesses, resulting in serious losses of state-owned assets," Xinhua said in a report published shortly after midnight.

The decision to expel Zhou was made on Friday after attendees at a meeting of the Political Bureau of the party's Central Committee deliberated over an investigation report on Zhou, who was in charge of China's massive domestic security apparatus before his retirement in 2012.

Prosecutors then announced his formal arrest and opened a criminal case against him early on Saturday.

The investigation found that Zhou had "seriously violated the Party's political, organisational and confidentiality discipline," the report said.

"Zhou leaked the Party's and country's secrets," the report went on to say.

"He seriously violated self-disciplinary regulations and accepted a large amount of money and properties personally and through his family. Zhou committed adultery with a number of women and traded his power for sex and money."

Once untouchable

By targeting Zhou, Xi showed the considerable power he has amassed since he took the helm of the party in November 2012.

Former members of the powerful Politburo Standing Committee had long been considered off-limits for prosecution in an unwritten rule aimed at preserving party unity.

But Xi vowed to go after both low and high-level officials in his campaign to purge the party of corruption and other wrongdoing that has undermined its legitimacy in the public eye.

"What Zhou did completely deviated from the Party's nature and mission, and seriously violated Party discipline. His behaviors badly undermined the reputation of the Party, significantly damaged the cause of the Party and the people, and have yielded serious consequences," the report said.

Zhou was once perceived as untouchable, with expansive patronage networks covering the sprawling southwestern province of Sichuan where he was once party boss and controlled the state oil sector, police and courts.

More significantly, as China's security chief, he oversaw the country's domestic spy agencies, a position that afforded him access to information on other high-ranking politicians who might pose a threat to him.