The other members were Bo, former Central Military Commission vice-chairman Xu Caihou, and Ling Jihua, a one-time top aide to former president Hu Jintao .

But for all the invective, the authorities seem to have held back from pressing the most serious political charges against him, restraint that analysts say may be to avoid too much rattling of the political system.

Zhou will stand trial for taking bribes, abuse of power and leaking state secrets. Of these, "intentionally leaking state secrets" might be the most sensational accusation but the Supreme People's Procuratorate's brief statement on the charges did not say who Zhou gave classified information to.

Mo Shaoping, a Beijing-based lawyer, said the state secrets charge fell under a broader category of crime known as "dereliction of duty", which applied to government employees who intentionally gave classified information to unauthorised recipients. It carries a jail term of up to seven years.

Mainland media also suggested that the state secrets charge was likely related to Zhou's close ties with Bo. A report said Zhou might have given his associates information on personnel arrangements before they were announced at the party's 18th National Congress in November 2012. He might have also leaked economic secrets to others for huge profits, it said.

The report also alleged that in 2012, Zhou tipped off Bo that Bo's right-hand man, police chief Wang Lijun, had sought asylum at the US consulate in Chengdu - the move that blew open the scandal surrounding the Chongqing party boss.

According to the report, Zhou and Bo had a secret meeting where they advocated "adjusting" the reform and opening-up policy initiated by former leader Deng Xiaoping in the late 1970s. The two men "celebrated their political rapport" and vowed to "play a big game".

Zhang Lifan, an independent party historian based in Beijing, said it was not surprising that Zhou was not charged with more serious crimes, given the language used to label him. Allegations such as "factionalism" or "involvement in unapproved political activities, were "obviously used in internal party disciplinary inspections, and do not have corresponding crime charges". "But if the authorities want to press harsher charges against Zhou, there are always options such as inciting subversion of state power, or inciting a split in the country," Zhang said.

Supreme People's Court president Zhou Qiang said last month that Zhou was among 28 disgraced officials who would face "open trials according to the law". But the charge of "leaking state secrets" would give the authorities a good excuse to close, or at least partially close Zhou's trial to the public to avoid embarrassment, Zhang said.

"An open trial usually means an under-the-table deal has been reached that the defendant will plead guilty in return for a more lenient sentence," Zhang said. "But in his open trial, Bo broke that agreement and defended himself, creating a lot of embarrassment for the party."

Among the three charges Zhou faces, only the bribery one can result in a death sentence. It is not clear if Zhou will face the death penalty, but the procuratorate said Zhou's corruption spanned his time as petroleum chief, Sichuan party boss and security chief, as well as his membership of the all-powerful Politburo Standing Committee.

Zhou "took advantage of his positions to seek gain for others, illegally accepted massive amounts of wealth from others. And [Zhou's] abuse of power has led to great losses of public funds and has done severe damage to the national and the public interest, causing adverse social impact," the Supreme People's Procuratorate said.

At least 37 companies, reaching as far afield as North America, are either owned by Zhou's family or connected to it, according to corporate documents reviewed by the . The businesses include oil production, property development, hydropower and tourism. Reuters reported earlier that the total amount of assets held by the family could be as much as 90 billion yuan (HK$114 billion), although others expressed doubts over the figure.

But it is likely that only a small amount of the alleged wealth, if any, can be linked directly to Zhou. As head of the family, Zhou did not get directly involved in business deals. The name "Zhou Yongkang", for example, does not appear in the thousands of pages of corporate documents seen by the . His elder son Zhou Bin , 42, is the one who holds strings of the business empire. Zhan Minli - Zhou Bin's mother-in-law - is the other key player. Zhou's nephew Zhou Feng and his sister-in-law Zhou Lingying also played a part. Together, these family members hold the vast commercial enterprise.