BEIJING • 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 China's President Xi Jinping will cement his grip on power.

Mr Sun Zhengcai, 53, had been party chief of the south-western megalopolis of Chongqing, until an abrupt announcement on Saturday morning that he no longer had the position and had been replaced by a rising political star close to Mr Xi.

The announcement, carried by state news agency Xinhua, did not say Mr Sun had a new position or use wording to suggest he was waiting for a further appointment.

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 was 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, referring to the party's 25-member decision-making body.

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 investigation but not formally charged. However, once a party announcement about a probe is publicly announced, they are almost always punished.

The Communist Party's anti-corruption watchdog, the Central Commission for Discipline Inspection, did not respond to a request for comment yesterday. Calls to the Chongqing government information office also went unanswered.

It was not possible to reach Mr Sun and not clear if any limits have been put on his movements.

Hong Kong and Taiwanese media as well as The Wall Street Journal have also reported that Mr Sun is being investigated. If confirmed, it would be the first probe of a sitting Politburo member, a key leadership body in the Communist Party, since the downfall in 2012 of another one-time Chongqing chief, Bo Xilai, later sentenced to life in prison for corruption and abuse of power.

Mr Sun did not appear on Chongqing television's Saturday evening broadcast when new party boss, Mr Chen Min'er, 56, was introduced to city officials by Mr Zhao Leji, head of the powerful organisation department which oversees personnel. Mr Zhao announced the probe at the Chongqing meeting, the first source added.

When Beijing's new party boss Cai Qi was unveiled in May, not only did Mr Zhao introduce Mr Cai but his predecessor Guo Jinlong was also present at the meeting and gave a speech, according to the Beijing city government's account of the event.

A third source with ties to the leadership emphasised how unusual it was that Mr Sun was not present or mentioned in the meeting where Mr Chen was presented as the new Chongqing leader. The source said the Chongqing meeting was told that Mr Sun had committed "political mistakes".

The three sources, who all spoke on condition of anonymity as they were not authorised to speak to foreign media, said Mr Sun was currently in Beijing.

Chongqing is perhaps best known for its association with its disgraced former party boss Bo, once a contender for top leadership before being jailed for life in 2013 in a dramatic corruption scandal.

Mr Sun was once seen as a potential premier, but sources say he has been out of favour after the party's anti-corruption watchdog in February criticised the Chongqing authorities for not doing enough to root out Bo's influence.

Diplomatic sources say Mr Sun had been scheduled to go on a trip to Europe recently but it had been cancelled. However, another candidate for party elevation, Guangdong party boss Hu Chunhua, visited Britain, Ireland and Israel last month. Such trips are often used by senior officials to raise their international profiles ahead of promotions.

Mr Sun's last public appearance as cited by state media was on July 12, at a Chongqing party meeting.

REUTERS, BLOOMBERG