China's ruling Communist Party said Tuesday it has launched an investigation into former security chief Zhou Yongkang -- one of its most powerful figures -- in a bold move in the country's deepening anti-corruption crackdown.

With the official announcement of the long-rumoured probe, Zhou becomes 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.

An official statement said Zhou, who retired from China's all-powerful Politburo Standing Committee (PSC) in 2012, is under investigation for "serious disciplinary violation" -- a term usually used to refer to corruption.

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, as PSC members have long been regarded as untouchable even after retirement.

Zhou was seen as a patron of fallen political star Bo Xilai, whom 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, China's official Xinhua news agency reported.

Analysts said the move shows that 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.

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.

However in a commentary late Tuesday, Xinhua said the announcement of the investigation "has revealed the courage and resolution of the Communist Party of China to purify itself and run itself with strict discipline".

"While putting an end to the months-long speculation and hearsay about Zhou's case, the announcement also clearly terminated a myth among many people that senior leaders are regarded to be immune from... Party discipline and the country's law enforcement."

- 'Nab the tiger' -

For months, allies of Zhou in his power bases in the southwestern province of Sichuan and China's state-owned oil giant CNPC have been targeted one by one by the ruling party's internal watchdog.

More than a dozen officials have been targeted in the sweeping probe into Zhou, with speculation mounting that the former security tsar himself was soon to fall.

Earlier this month, three associates of Zhou were stripped of their Communist Party membership.

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

State-run broadcaster China Central Television (CCTV) mentioned the Zhou probe 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 evening, 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.

"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."