The Chinese Communist Party's anti-graft watchdog has launched an investigation into a Politburo member once seen as a contender for a top leadership post, just months before a major political congress.

The Central Commission for Discipline Inspection is investigating Sun Zhengcai, who some 10 days ago was ousted as party chief in the major city of Chongqing, for "serious discipline violation", the official Xinhua news agency said.

Sun is the first serving member of the 25-person Politburo to be placed under investigation since Bo Xilai, also a former Chongqing party chief who was jailed for life in 2013 in the wake of President Xi Jinping's anti-corruption campaign.

Xinhua's brief report did not expand on the accusations nor say whether Sun would remain a member of the Politburo. His replacement as party chief in Chongqing was announced on July 15.

Analysts say the shake-up in Chongqing will have implications for the Communist Party's 19th congress later this year, which will likely cement Xi's position as the most powerful Chinese leader for a generation.

At the gathering, Xi is widely expected to secure a second five-year term.

But all eyes will be on who will depart or ascend to the Politburo's standing committee, the elite group of seven politicians who run the world's second largest economy.

Sun, who was replaced in the city by former Xi aide Chen Miner, had once been tipped for promotion to the standing committee. At 53, he is the youngest member of the Politburo.

- 'Threat' to Xi's agenda? -

Sun, who had kept a low profile, had even been touted as Xi's potential successor, said Simone van Nieuwenhuizen, a Sydney-based researcher and co-author of "China and the New Maoists".

"His downfall signals a growing sense of insecurity among Xi and his followers," van Nieuwenhuizen told AFP.

While the exact reasons for his fall are unknown, "we do know however that Sun had been promoted as a protege of Xi's predecessor Hu Jintao," she said.

"Xi could therefore consider Sun a threat to his own agenda and political ambitions; other proteges of Hu have also previously been held back from seeking higher positions under Xi."

At least four new standing committee members will likely be named at the congress and "Xi is overseeing their selection", said Bill Bikales, head of Bikales Advisors, an East Asian political and economic advisory service.

"Xi Jinping is seizing personal control of the succession process in a way that neither (former president) Jiang (Zemin) nor Hu was able," he said.

"This is unprecedented in the post-Deng Xiaoping decades and has implications for the 20th Congress in 2022 as well," Bikales said, referring to the nation's longtime leader who died in 1997.

As the country's fourth largest city and an industrial metropolis in the southwest of the country, Chongqing is a major post for ambitious politicians.

Bo was rumoured to be part of a network plotting to stop Xi from assuming control of the party in 2012, and Sun was tasked with erasing Bo's influence on the city.

But in February party discipline inspectors publicly rebuked Sun for failing to get the job done, leaving his future in question.

Shortly after replacing Sun, Chen was quoted as saying by a local daily that the city must "resolutely remove the bad impact" of Bo's case.

Sun's downfall has raised Chen's profile.

Chen first worked with Xi in 2002, when the latter became party chief of the eastern province of Zhejiang.

In late 2012 the former propaganda chief's career kicked into overdrive following Xi's ascension to the head of the Communist Party.

Chen became governor of the southwestern province of Guizhou in 2013, adding the title of party secretary in 2015.

Source: AFP