Disgraced Chinese politician Bo Xilai has been placed under criminal investigation, state media said Friday, after he was expelled from the country's parliament and stripped of his legal immunity.

The announcement follows intense speculation on the fate of the former party boss of the southwestern city of Chongqing in the lead-up to a once-in-a-decade leadership transition set to begin on November 8.

State news agency Xinhua said the Standing Committee of the 11th National People's Congress (NPC) removed Bo from his post late last month, but made the announcement only on Friday at a bi-monthly session.

"According to the law on the deputies to NPC and to local people's congresses, his post was terminated," Xinhua said, quoting a statement from the standing committee at the end of a four-day meeting.

The report also said that Bo had "borne major responsibility" for the murder of a British businessman that resulted in his downfall, without elaborating.

A later Xinhua report said the Supreme People's Procuratorate had "decided to put Bo Xilai under investigation for alleged criminal offences, as well as impose coercive measures on him in accordance with the law".

"The investigation is under way," Xinhua, in a brief English-language report, quoted a statement as saying.

The focus will now shift to when Bo will face trial for a litany of alleged crimes, with most commentators suggesting the court case will be after the Communist Party Congress next month at which new leaders will be selected.

Li Xiaolin, a lawyer who had been appointed by the Bo family, told AFP Friday that he did not expect the court case to be held until after the congress.

Party chiefs are set to meet for the Communist Party Central Committee's 7th Plenary Session on November 1 to formally approve the convening of the congress and confirm the decision made last month to remove Bo from the party.

Bo's expulsion from the NPC comes after state media announced last month that he would "face justice" for alleged abuse of power, taking bribes and improper sexual relations.

This was seen as an unprecedented public rebuke for a senior Chinese party official as authorities looked to lay to rest a damaging episode that shocked China and saw Bo's wife convicted of murder.

Bo, the party boss of the southwestern metropolis of Chongqing, had been seen as a candidate for promotion to the party's top echelons.

But he was brought down earlier this year by murder allegations against his wife Gu Kailai that came to light when Bo's key aide and police chief Wang Lijun sought refuge in the US consulate and detailed a string of alleged crimes.

His wife was handed a suspended death sentence -- commonly commuted to a life sentence -- for fatally poisoning British businessman Neil Heywood.

The Xinhua report also said Bo -- who is expected to face a lengthy prison sentence -- had "seriously violated CPC (Communist Party Congress) disciplines".

Bo was earlier removed from his Chongqing post, with analysts saying the affair exposed deep divisions within the party as he retains a large following among left-leaning members.

In response to the opening of the NPC meeting this week, hundreds of Bo's supporters urged the parliament in an online petition not to expose him to a potentially unfair trial.

"The entire trial involving the Bo case has the problems of facts that are unclear, evidence that is neither reliable nor adequate and procedures that are not lawful," the letter on the leftist Red China website said.

There were more than 500 signatories in support of Bo, who had championed a "red revival" before his downfall.

The Communists had hoped for a smooth build-up to a congress that is tightly scripted to underline the party's claim to be the only legitimate force capable of ruling the world's most populous nation.

But the party has instead been rocked by the Bo case and the details of murder, million-dollar deals and the affluent lifestyles of the Communist Party power elite that it laid bare.

The congress typically lasts about one week and ends with the traditional unveiling of a new Politburo line-up that this year is expected to see Vice President Xi Jinping promoted to Communist Party general-secretary.

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