BEIJING: China's ruling Communist Party has put former security chief Zhou Yongkang, one of its most powerful men, under investigation, it said Tuesday, the most senior official to fall for decades.

Zhou, who retired from China's all-powerful Politburo Standing Committee (PSC) in 2012, is being probed for “serious disciplinary violation”, the ruling party's Central Commission for Discipline Inspection (CCDI), its internal watchdog, said in a statement.

The term is usually used to refer to corruption.

The official announcement of the probe, which has been speculated about for months, breaks a long-held principle that top officials are untouchable.

Zhou is the most senior member of the Communist Party to be investigated since the infamous Gang of Four, a faction that included the widow of founding leader Mao Zedong, were put on trial in 1980.

Zhou was seen as a patron of fallen political star Bo Xilai, who he is said to have backed for a slot on the PSC, but whose career imploded after the death of a British businessman, for which Bo's wife was convicted of murder.

The decision to investigate Zhou was made in accordance with the ruling party's constitution, and the “discipline inspection authority's case investigation regulation”, China's official Xinhua news agency reported.

The decision will have been preceded by extensive negotiations within the factionalised ruling party, but is still likely to send shockwaves through the political establishment.

Analysts said that the move shows party chief Xi Jinping has now amassed enough power to break even longstanding taboos in his much-publicised anti-corruption sweep.

“There is an unwritten rule that they will not go after former members of the Politburo Standing Committee,” said Willy Lam, a politics specialist at the Chinese University of Hong Kong.

“The party elders like Jiang Zemin and Li Peng and so forth were opposed to incriminating Zhou Yongkang,” he said, referring to China's former president and premier.

“It shows that Xi Jinping is powerful enough or resourceful enough to convince the party elders,” he added.

Strike hard

Xi has vowed to crack down on endemic graft among top party members, or “tigers”, as well as low-ranking members, or “flies”, but critics say he is unlikely to succeed without more fundamental reforms such as greater press freedoms and independent courts.

Joseph Cheng, professor of political science at City University of Hong Kong, said the move demonstrates Xi “understands that striking hard at corrupt leaders will make him a very popular leader, will help him consolidate his base of power”.

News of the probe came as Communist Party authorities announced that their next major political meeting, known as the Fourth Plenum, will be held in October and focus on “key issues concerning the rule of law”.

Zhou Yongkang's son, Zhou Bin, has also reportedly been arrested in Yichang in central China for suspected “illegal business operations”, respected financial magazine Caijing said.

Calls by AFP to the Yichang municipal procuratorate went unanswered, and state-run media had not reported on the younger Zhou as of late Tuesday.

'Nab the tiger'

For months, allies of Zhou in his powerbases in the southwestern province of Sichuan and China's state-owned oil giant CNPC have been targeted one by one by the CCDI, with more than a dozen being picked off.

Earlier this month, three associates of Zhou, Ji Wenlin, Zhou's former secretary; Yu Gang, an ex-vice director of the office of the Central Politics and Law Commission (CPLC); and Tan Hong, formerly of the public security ministry, were stripped of their Communist Party membership.

Yet China's state-run media steadfastly refrained from mentioning Zhou by name until Tuesday, and the Communist Party's vast censorship apparatus swiftly swept away any use of his name on the country's popular social networking sites.

State broadcaster China Central Television (CCTV) mentioned the Zhou probe only as the fourth item during its evening news broadcast, with an anchor taking less than a minute to read a brief version of the Xinhua report.

Even after authorities made the announcement Tuesday, searches for “Zhou Yongkang” on Sina Weibo, a Chinese Twitter equivalent, initially returned a message saying that results were not displayed “according to relevant laws, regulations and policies”.

The site later unblocked his name, and social media users quickly weighed in on the announcement: an hour after the news was first reported, “Zhou Yongkang” was the most-discussed topic on Sina Weibo, with 1.2 million postings.

Commenters have long used noodles as a way to refer to Zhou while avoiding censors' attention, playing on a popular brand, Master Kang.

“Finally, the noodles that have been cooking for so long are coming out of the pot,” one Weibo user wrote.

“There are hidden reasons behind it,” mused another.

“We support you, Chairman Xi,” another wrote. “You finally nabbed the tiger. “