Sun Zhengcai, the former Communist Party chief of the Chinese city of Chongqing, is under investigation by authorities, the Wall Street Journal has reported, citing people it didn't identify.

Sun, 53, was the youngest member of the party's Politburo and was considered a rising star ahead of a key leadership reshuffle later this year. State media reported earlier Saturday that he was replaced by a provincial party chief — Chen Miner — who is an associate of President Xi Jinping.

A party meeting in Chongqing to announce Sun's departure was told he was under investigation, the Journal reported Saturday, adding that no details of the probe were given. Chongqing is a fast-growing mega-city and one of four centrally administered municipalities alongside Beijing, Shanghai and Tianjin.