Ling Jihua, head of the ruling Communist Party's United Front Work Department and national political advisor, is under investigation for "suspected serious disciplinary violation", Xinhua news agency reported today, without giving further details.
The authorities said the 58-year-old leader was being investigated for disciplinary violations, which usually refers to corruption.
The Communist Party of China (CPC) Central Commission for Discipline Inspection, the anti-graft body, announced the investigation in a brief statement today.
According analysts, the investigation against Ling shows that President Xi is secure enough to probe even the trusted advisers of his predecessor Hu. They also say the action against Ling could be a setback to the former president.
Ling, who was a personal aide to Hu, found himself in the media spotlight and mired in controversy after his son Ling Gu died after crashing a Ferrari in Beijing with two nude or half-dressed women in March 2012.
Ling was accused of covering up the scandal.
Many of Ling's other associates and relatives, including another brother Ling Zhengce, are also being investigated for graft charges.
It is to be seen whether investigations against Ling and subsequent prosecution will implicate other senior officials also.
Several senior Chinese officials, including some of the top Generals of the Chinese military, have been held for corruption in the anti-graft campaign initiated by President Xi in the last one year.
Xi's campaign aimed at restoring the sagging image of over six decades of CPC rule for the first time started targeting top retired leaders discarding long standing practice of not to prosecute those superannuated officials.
Zhou was the ninth-ranking member in the nine-member Standing Committee of the CPC headed by former President Hu.