BEIJING (BLOOMBERG) - Mr Sun Zhengcai, the former Communist Party chief of the Chinese city of Chongqing, is under investigation by authorities, the Wall Street Journal reported on Saturday (July 15), citing people it did not identify.

Mr 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 on Saturday that he was replaced by a provincial party chief - Mr Chen Min'er - who is an associate of President Xi Jinping.

A party meeting in Chongqing to announce Mr Sun's departure was told he was under investigation, the Journal reported, 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.

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, who was later sentenced to life in prison for corruption and abuse of power.

It comes before a party congress where many of the 25 members of the Politburo are expected to be replaced as Mr Xi heads into the second half of his scheduled term in power.

Five of seven members of the supreme Politburo Standing Committee - all except Mr Xi and Premier Li Keqiang - are also scheduled to retire.

Mr Xi has for more than four years embarked on a sweeping crackdown on graft at all levels of the party. He has warned corruption is one of the biggest threats to the party's future. The campaign has also allowed him to amass greater power.

Mr Sun had previously drawn praise from the president. During a visit to Chongqing in January 2016, Mr Xi stood side by side with Mr Sun and admired various projects under his purview, according to the official Xinhua News Agency.

While inspecting port and railway projects to support Mr Xi's initiative for a new Silk Road trading route to Europe, the president said: "This place is very promising."

Still, the party's top anti-graft agency, the Central Commission for Discipline Inspection, issued a statement in February after an inspection trip to Chongqing saying the municipality fell short of "clearing toxic residue" from Bo's era. And in June, state media said Chongqing's top police officer He Ting had been relieved of his post.

The party's Organisation Department chief Zhao Leji presided over Saturday's appointment meeting and transition ceremony, where Mr Sun was absent, according to the Chongqing Daily.

"Sun's absence from the transition ceremony was highly unusual and is a good indication that he's in trouble," said adjunct political professor Willy Lam at the Chinese University of Hong Kong.

Mr Sun's removal could help Mr Xi solidify his grip on power ahead of the party reshuffle, Dr Lam said. "It's another way to demonstrate he's in full control of the succession game."

In June 2015, Zhou Yongkang, a retired member of the Politburo's supreme Standing Committee, was sentenced to life in prison, the highest-ranking official convicted for graft in party history.

Zhou, until 2012, oversaw China's vast security apparatus, including police, prosecutors and the courts. He held sway over a network of proteges and former aides, serving on the Standing Committee with Mr Xi before he retired and the latter was elevated to general secretary in 2012.

Mr Sun, together with Guangdong party boss Hu Chunhua, had been regarded as potentially in line to be elevated to the Standing Committee.

Instead, Mr Chen, 56, party chief of the southwestern province of Guizhou, was announced on Saturday as the new chief of Chongqing.

Prior to his Chongqing appointment, Mr Sun was party chief of the northeastern province of Jilin between 2009 and 2012, and was China's Agricultural Minister between 2006 and 2009 in the administration of Premier Wen Jiabao.

Mr Chen has crossed paths with Mr Xi during his career and the two have established links. He advanced to governor of Guizhou - his first provincial-level promotion - in December 2012, shortly after Mr Xi became overall party chief.

In July 2015, Mr Chen was promoted to provincial party chief, only the third regional party boss born in the 1960s.

As propaganda chief previously in Zhejiang, which Mr Xi led from 2002 to 2007, Mr Chen shepherded Mr Xi's regular column in the provincial newspaper. More than 200 of those columns were compiled into a book touted as the origins of Mr Xi's political philosophies.

Chongqing, a sprawling municipality of almost 30 million people, is located more than 1,400km from the coast and served as China's capital during the Japanese invasion. Chongqing has prospered as the country's economic boom rolled inland, reinventing itself as a regional hub for manufacturing, logistics, financial services and foreign investment.