The long-awaited official announcement that Zhou would face trial came at the beginning of last month.

He will be tried by a court in Tianjin, about 120km from Beijing, on charges of taking bribes, abuse of power and intentionally leaking state secrets. The maximum punishment for bribery is death; the state-secret and abuse of power charges each carry up to seven years in jail.

Zhou, who was detained in mid-2013, is undergoing so-called "shuanggui", an internal disciplinary procedure for Communist Party members, particularly those suspected of graft.

A source confirmed that his two sons - Zhou Bin and Zhou Han - had also been detained. Both have chosen their own lawyers with the authorities' approval, and at least one has been allowed to meet his lawyer in Hubei .

It is common for confessions that officials make during investigation to be used as evidence in court. One topic of speculation concerns whether Zhou has retracted any confessions he might have made during this period.

"Conventionally, cases involving senior officials usually go according to plan, as almost all have confessed to the charges prior to trial," said a lawyer with experience in such cases. "Taking back confessions in court wouldn't help much."

If Zhou felt the court was likely to hand him the death sentence, he might have decided to retract his statements so as to implicate other officials who might otherwise have gone free, said Beijing-based political commentator Zhang Lifan .

"He may see it as: 'If I am going to die, you won't live a good life either'," he said.

Another source of speculation surrounds Zhou's defence and whether he will represent himself. As the former head of the mainland's legal and security apparatus, he has deep insight into the criminal trial system. If the proceedings are open, mounting his own defence gives him a powerful platform.

In April, authorities reportedly choose Gu Yongzhong , a legal professor at the China University of Political Science and Law, to be Zhou's lawyer. Gu has refused to confirm this, and his acquaintances have refused to discuss the matter.

So far, at least a dozen of Zhou's top aides have been targeted in the investigation, but only two have been tried.

Jiang Jiemin, former China National Petroleum Corporation chief, admitted in a Hubei court last month to taking bribes, possessing assets from unidentified sources and abusing his position. His sentencing date has not been announced.

Li Chuncheng, former deputy provincial party chief of Sichuan - one of Zhou's power bases - pleaded guilty to accepting bribes and abusing his power less than two weeks later.

Prosecutors said that "under Zhou's suggestions, [Li] has violated related rules to help others gain illicit interests".

It was the first time Zhou's name had appeared in court documents released to the public.

He was referred to indirectly in the corruption trial of Bo Xilai , the former party chief of Chongqing, in 2013.

Bo testified he had received orders from the party's top law and order commission headed by Zhou in early 2012 on how to cover up the defection of his right-hand man, Wang Lijun. Bo was jailed for life.

Zhou is the first member of the supreme Politburo Standing Committee to be criminally charged, and whether the proceedings are open or not, the event will invite comparisons with the mainland's most famous trial - that of the "Gang of Four" member Jiang Qing.

Jiang, the wife of late leader Mao Zedong, was put on trial in the 1980s for crimes including counter-revolution. She refused to confess, and was sentenced to death, later suspended. After her release from prison on medical grounds, she killed herself in 1991.

As Jiang Qing's trial was televised and Bo's court proceedings were released in detailed transcripts on social media, there are growing calls for an open trial of Zhou to dispel concerns that the charges are politically motivated. But the charge of leaking state secrets may prevent the authorities from making Zhou's trial as transparent as Bo's.