Xu, 71, has been placed under - a Communist Party disciplinary measure used to interrogate party members suspected of corruption - in a secretive detention centre.

If the party disciplinary agency gathers ample evidence against him and the case goes to court, Xu would become the highest-ranking military officer to go on trial for corruption. Analysts said the trial would be held in a military court, which means the verdict would not be publicly announced

Xu was promoted to the CMC in 1999 and became its vice-chairman in 2004. Rumours about his fate were fuelled when he was absent from last year's National People's Congress, where he formally retired from the CMC. Over the past decade, Xu wielded huge influence over the world's largest military as former president Hu Jintao, the nominal commander-in-chief, was known for his weak control of the army.

Xu's detention means the top leadership will soon announce the results of the graft inquiry into the army's former deputy logistics chief, Gu Junshan. Gu, one of Xu's closest subordinates, has been under since early 2012.

The disciplinary decision against Xu, who has bladder cancer, was only made very recently by the top leadership after he was believed to have been let off the hook because of his terminal illness. His supporters in the army had been pushing for leniency, arguing that the diagnosis of terminal cancer early last year was a fate equal to the death penalty.

They suggested the precedent of the late vice-premier Huang Ju, who was spared a corruption probe due to his pancreatic cancer even though he was involved in various corruption cases, could also be applied to Xu.

But the prospect of a lack of action against Xu caused widespread anger among reformists in the military, who accused the general of rampant corruption while in power.

"When Xu was taken away for investigation, many mid-ranking cadres like me were very happy," a Beijing-based retired senior colonel told the .

"We believed at the time that Xu would be 'the biggest tiger' that would be caught in the anti-graft campaign. We were very disappointed to learn that Xu was exempt [from prosecution] because of his illness. There is much concrete evidence to prove Xu's involvement in corruption.

"Many military officials, especially at the middle-level, couldn't accept Xu's immunity from a graft probe."

The ambiguity surrounding Xu has caused the case of Gu, one of his closest allies, to drag on for nearly two years without trial.

A source told the that Gu had shared a substantial part of his bribes with Xu. He cited the example of a 20 million yuan(HK$25.3 million) debit card that Gu gave to Xu's daughter as a wedding gift several years ago.

Another Shanghai-based senior colonel said that during Xu's tenure, the buying and selling of military ranks was widespread in the PLA, with Gu himself being promoted under the practice.

"Such transactions did not only happen at the highest levels, but also expanded to the grass roots," he said. "The PLA has improved retirement and job replacement for veterans in the past decade, but it [the selling of ranks] has become a cash cow for many senior officials."

He said he was told that even a farmer's child needed to pay at least 50,000 yuan to pass military recruitment tests.

The retired senior colonel said it was routine during Xu's tenure for military officials to pay if they wanted to climb the career ladder. "The more you could pay, the higher you could climb," he added.

The Beijing-based retired senior colonel said Xi Jinping, who is also chairman of the CMC, vowed to improve the PLA's public image after realising the alleged extent of the buying and selling of military ranks by Xu and his allies.

"It will be a tough job for Xi to deal with the corruption battle as Xu was in charge of high-level personnel arrangements from 2004 to 2013, with almost all the army's senior officials being promoted by him," he said.

"A major shake-up of top-brass in the army is inevitable in the future, but it takes time. In the near future, Xi might open a gate to Xu's supporters who are willing to straighten up and pledge loyalty to the party, to keep up the army's morale."

Xi has launched big anti-graft campaigns and told senior officials to take part in self-examination and self-discipline meetings.

Xu made his last public appearance on January 20 in Beijing at a Lunar New Year gala performance for retired military officials. During the interval, Xu approached Xi several times to speak but was ignored, said sources who were there.

Gu, who was in charge of the military's extensive property portfolio, reportedly received hundreds of millions of yuan in cash and gifts.

Sun Sijing, head of the anti-graft watchdog overseeing the army's general logistics department, confirmed Gu was being investigated and that details about the investigation would be announced soon.