BEIJING (Reuters) - The former Communist Party boss of one of China’s most important cities, the southwestern megalopolis of Chongqing, is under investigation for “suspected serious violations of discipline”, the party’s anti-corruption watchdog said on Monday.

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Sun Zhengcai, a senior official once considered a contender for top leadership, had been party chief of the city until an abrupt announcement this month that he had been replaced by a rising political star close to President Xi Jinping.

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

The Central Commission for Discipline Inspection (CCDI) announced the investigation into Sun in a one-line statement on its website, a move that comes ahead of a key party congress in the autumn where Xi will cement his grip on power.

Sources had earlier told Reuters that Sun was under investigation for discipline violations, a term that can encompass everything from taking bribes to not toeing the party line.

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.

It was not possible to reach Sun directly for comment.

Sun had been seen as a potential candidate for elevation at the autumn congress and as a possible future premier. But sources with ties to the leadership and foreign diplomats say Sun has been out of favor after the CCDI in February criticized Chongqing authorities for not doing enough to root out Bo’s influence.

At that meeting, Sun said he accepted the watchdog’s assessment “without question”, according to a party statement at the time.

Sun was unceremoniously replaced in Chongqing by Chen Miner, who days after taking up the post demanded officials banish the “evil legacy” of Bo.

Chen is a rising political star close to Xi, and is seen as a potential new member of the party’s elite Standing Committee when it is unveiled after a reshuffle at the once-every-five-years party congress.

Since assuming power in late 2012, Xi has pursued a relentless campaign against corruption, warning that the problem could threaten the party’s ability to retain power, though some analysts say he is also taking down political rivals.