Enjoying the patronage of party elders such as former president Jiang Zemin and former vice-president Zeng Qinghong, Mr Zhou scaled the party ranks to claim a seat on the all-powerful Politburo Standing Committee, having served stints as party secretary of the south-western province of Sichuan and as a senior executive of the state-owned China National Petroleum Corporation (CNPC). In short, he was untouchable - a calculating politician whose tentacles extended to some of the Communist Party's most influential organs. But for months now, Mr Zhou has been under a cloud, weakened by his support for disgraced Chongqing party chief Bo Xilai, who fell from office after his wife, Gu Kailai, was found to have murdered British businessman Neil Heywood. One of the many salacious details to emerge from Mr Bo's trial was his claim that it was Mr Zhou who ordered the US consulate in Chengdu to be surrounded by police cars, in an effort to flush out fugitive Chongqing police chief Wang Lijun and prevent him passing information about Mr Heywood's murder to US officials. Before his fall, Mr Bo had been tipped to take Mr Zhou's spot on the Politburo Standing Committee.

Instead, he is all but certain to receive a heavy sentence after being tried on charges of corruption and abuse of power last month. The verdict is expected within days. All the while, the net is closing around Mr Zhou, who has emerged as the prize scalp of new President Xi Jinping's anti-corruption drive. One by one, Mr Zhou's most trusted aides are falling and his power base is being pulled from beneath him. Jiang Jiemin, a long-serving chairman of CNPC before being shifted to head up the government organ overseeing state-owned enterprises in March, is a Zhou protege. So, too, is Wang Yongchun, a senior CNPC executive. Both were among four senior executives stood down last month and investigated for ''severe discipline violations'', a party euphemism for corruption.

On Thursday, Jin Jianping, the head of gas firm Tianjin Jinran, was arrested before he tried to board a train in Hebei province. Also detained are Zhou allies from his Sichuan days. Li Chuncheng, a deputy party chief in the south-western province, was arrested in December, while a former deputy governor, Guo Yongxiang, was arrested in June. Businessman Wu Bing, said to be one of Mr Zhou's closest associates, is also in detention. The string of arrests has led many to believe the announcement of Mr Zhou's is only a matter of timing, to be fitted in somewhere between the Bo verdict and a key leadership plenum in November. Indeed, the Financial Times has reported that Mr Zhou is already under house arrest. ''Much depends on how much Jiang Jiemin and others confess,'' said Chen Ziming, a political analyst and former Tiananmen Square activist. ''If they reveal direct links to Zhou or his son or family, investigations might even start before the [November] plenum.''

One source with ties to China's political elite said Mr Zhou was ''surely'' in trouble and that an official investigation would be announced ''soon'', with some big names in Mr Zhou's other stronghold, the Public Security Bureau, likely to be indicted. But others were more sceptical. ''At least for the moment, Zhou is not under investigation, but rather is assisting with investigations,'' political commentator Li Weidong said. Another source with ties to the leadership said Mr Zhou had too much dirt on senior party figures to be brought down. ''You think it's so easy to touch Zhou Yongkang?'' he said. The stakes are high for Mr Xi, in what could be an early defining moment for his leadership.

Pursuing Mr Zhou breaks a decades-old unwritten agreement not to go after a standing committee member, past or present. But Mr Xi has identified the fight against corruption as a key route to winning back support with a public increasingly disillusioned by endemic graft and wealth inequality in Chinese society. ''If Xi doesn't win the people's hearts and minds, someone else will,'' Chen Ziming says. Mr Bo was seen by many as a direct rival to Mr Xi and as a threat to the overall cohesion of the Communist Party, in no small part because of his boundless ambition and attempt to revive symbols and rhetoric of the Mao era. But with Mr Bo purged, Mr Xi has disappointed liberal intellectuals by his own apparent ideological lurch to the left.

Even as China takes what appear to be genuine strides towards economic reform, internal warnings have confirmed Mr Xi's fears that the Communist Party is vulnerable to growing public discontent. An internal memo, referred to as Document No. 9, outlined seven taboos that would be forbidden from public discussion, including democracy, the ''universal values'' of human rights and a free press, as well as criticism of the party's turbulent past. ''Western forces hostile to China and dissidents within the country are still constantly infiltrating the ideological sphere,'' the document said, according to a copy obtained by The New York Times. Chinese universities have been ordered to steer clear of the seven taboos in their teaching. Prominent Peking University lecturer Xia Yeliang, a signatory to Charter 08 - the call for personal freedoms that landed Nobel laureate Liu Xiaobo in jail - has been threatened with the sack for refusing to oblige. ''The most important thing [from Mr Xi's perspective] is to strike with both fists,'' Chen Ziming says. ''One fist to strike out at corruption and the other to hit out at intellectuals.''

In a throwback to Mao's era, in June Mr Xi called for party cadres to pursue ''the mass line'' and further their ties with ordinary people. Visiting a village from which Mao attacked Beijing in 1949, Mr Xi said: ''Our red nation will never change colour.'' Even more striking has been a push to curb internet forums. In a meeting of 30 or more propaganda chiefs while the Bo trial rolled on, Mr Xi demanded his party regain control of new media. In recent weeks, censors have increased controls online and police have stepped up harassment of liberal intellectuals and activists. ''I've been followed very closely for two days now,'' one told Fairfax Media via text message. ''They are pulling out all the tricks.''

Chinese-American businessman Charles Xue, who often posts on social issues to his 12 million followers on Weibo - a broad equivalent of Twitter - was made an example of. Arrested for soliciting prostitutes, he has been in detention for more than three weeks and appeared on state television making a grovelling apology for his behaviour online. Even if Zhou Yongkang does fall, his authoritarian legacy appears to be in safe hands.