- Sun Zhengcai, a member of the Communist Party's Politburo, is suspected of "serious violation of party discipline", according to a source.
- Sun was replaced by Guizhou's party chief, Chen Miner, a trusted protégé of President Xi Jinping.
- Analysts said Chen was now better positioned to ascend to the top ranks and become a possible successor to Xi.
A top contender for a role in China's next leadership is being investigated by the anti-graft agency of the Communist Party, according to people familiar with the matter.
Sun Zhengcai, at 53 the youngest member of the party's 25-strong Politburo, is suspected of "serious violation of party discipline", according to a source in Chongqing.
He was removed as party boss of the city on Saturday.
The development could have a significant impact on the upcoming leadership reshuffle in the autumn. Sun was seen as having a strong chance of being named to the Politburo Standing Committee.
Sun was now in the middle of an investigation "at the conversation level", the city's propaganda chief told senior editors in Chongqing on Saturday, according to the source.
The internal briefing did not state whether he had been placed under official investigation.
Sun was replaced by Guizhou's party chief, Chen Miner, a trusted protégé of President Xi Jinping.
Sun was not present at the handover announcement, in a break with protocol that adds to evidence that he is the subject of an investigation.
Another source from Chongqing said the handover ceremony was abruptly announced the night before, after the city's major leaders had finished work.
A third source said earlier that Sun had been taken away by the party's Central Commission for Discipline Inspection.
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Sun was seen attending a high-level finance work meeting in Beijing held on Saturday, video footage of state-run CCTV shows.
He was seated with other Politburo members but was the only one not seen in a close-up shot.
Regarded by some as a protégé of former premier Wen Jiabao, Sun had been viewed as a front runner for a seat on the Politburo Standing Committee at the upcoming 19th national congress.
The party boss of Chongqing, one of China's four largest cities, traditionally sits on the Politburo.
Chen is a close associate of Xi, having served as his propaganda chief in Zhejiang province.
Analysts said Chen, 56, was now better positioned to ascend to the top ranks and become a possible successor to Xi.
"He is now [likely] very hopeful of entering the Politburo, and it's even possible that he would replace Sun as a leading contender for the top job," Beijing-based political commentator Zhang Lifan said.
Chen Daoyin, an associate professor at Shanghai University of Political Science and Law, said Chen could end up following the same career trajectory as Xi, who was named Shanghai party boss shortly before he was promoted to president-in-waiting in 2007.