Image copyright Reuters Image caption Sun Zhengcai was abruptly replaced as Chongqing party chief earlier this month

Chinese authorities say they are investigating a top politician once seen as a candidate for the highest ranks of the leadership.

Sun Zhengcai was the Communist Party chief of a major metropolis, Chongqing.

Before his abrupt removal from office earlier this month, he was a strong contender to rise to the elite seven-member committee that rules China.

But now China's anti-corruption body says it is investigating him for serious violations of discipline.

The move comes ahead of a key Communist Party meeting in the autumn when those tipped to succeed the current president and premier - who according to convention will step down in 2022 after 10 years in power - are expected to be revealed.

Murder in the Lucky Holiday Hotel

Mr Sun is the youngest member of the Politburo, the 25-member body subordinate only to the seven-member Standing Committee.

He is the first serving Politburo member to be investigated since Bo Xilai, a charismatic high-flyer who also served as Chongqing party chief until he was jailed for corruption in 2013 amid a scandal over the murder of a British businessman.

'Shadowy world' - Analysis by Celia Hatton

One of the world's most important elections is happening right now, behind closed doors.

In a few months, China is expected to unveil its next generation of leaders. Until last week, Sun Zhengcai was expected to be one of them but then he was abruptly removed from his job and now, he's under investigation.

A few weeks ago, Mr Sun publicly professed his continued loyalty to Chinese President Xi Jinping. Perhaps Mr Xi questioned that loyalty, or other political machinations could have led to Sun Zhengcai's surprising downfall. The real reasons that led to his demise are hidden from view.

But two things are clear: first, the leadership race is proving to be more turbulent than predicted. And also, in the shadowy world of Chinese politics, anything can happen before an official announcement is made.

Mr Sun was appointed party leader in Chongqing in November 2012, having previously served as party secretary in Jilin province and as agriculture minister from 2006-2009.

He has now been replaced in Chongqing by Chen Min'er, who is seen as a protégé of President Xi.

Since taking power in 2012, Mr Xi has launched a wide-ranging crackdown on corruption which has brought down several senior officials and which, correspondents say, he is believed to be using to consolidate power ahead of the party meeting in the autumn.