Sun Zhengcai, the former head of the party in Chongqing, was once seen as a future Premier

A senior Chinese official who was considered a contender for top leadership has been put under investigation, three sources with ties to the leadership said, ahead of a Communist Party congress in the autumn where Xi Jinping will cement his grip on power.

Sun Zhengcai had been party chief of the southwestern megalopolis of Chongqing, until an abrupt announcement on Saturday morning that he no longer had the position and had been replaced by a rising political star close to President Xi. The announcement, carried by state news agency Xinhua, did not say Mr. Sun either had a new position or use wording to suggest he was waiting for a further appointment.

‘Still a comrade’

A source who has been briefed on the matter said Mr. Sun is suspected of “serious discipline violations”, a term that can encompass everything from taking bribes to not toeing the party line. The source added that it was a “conversation investigation”, meaning it’s not yet at the stage of a formal probe.

A second source with ties to the leadership told Reuters that Mr. Sun is undergoing investigation for suspected “violation of political discipline”. The source declined to elaborate. “But he is still a comrade. He is still a Politburo member,” the source said. Officials are stripped of their title “comrade” once a formal legal case is filed against them and they are expelled from the party. Officials are sometimes put under probe but not formally charged. However once a party announcement about a probe is publicly announced they are almost always punished.

New boss

Hong Kong and Taiwan media as well as the Wall Street Journal have reported Mr. Sun is being investigated. Mr. Sun did not appear on Chongqing television’s Saturday broadcast when new party boss, Chen Miner, was introduced to city officials by Zhao Leji, head of the powerful organisation department which oversees personnel decisions. Mr. Zhao made the announcement about the probe at the Chongqing meeting, the first source added.

Chongqing is perhaps best known for its association with its disgraced former party boss Bo Xilai, once himself a contender for top leadership before being jailed for life in 2013 in a dramatic corruption scandal. Mr. Sun had been seen as a potential candidate for elevation at the autumn congress and as a possible future Premier, but his star has waned.