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.