Sun, 53, was removed from his post as party chief of Chinese mega-city Chongqing in July.
The Supreme People's Procuratorate (SPP) of China announced today that it has started proceedings against Sun for suspected bribery and had approved the taking of "coercive measures" against him, state-run Xinhua news agency reported.
The measures against Sun may include summons by force, bail and detention, and further investigation is under way, the report said.
He was removed ahead of the once-in-a-five-year-congress of the CPC, which endorsed a five-year term for Xi.
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Sun was the youngest member of the 25-member Politburo and he was even seen in some quarters as Xi's potential successor.
Ahead of the Congress, Liu Shiyu, chairman of the China Securities Regulatory Commission said Xi had foiled the coup plot by former political heavy weights who were at the receiving end of his high-profile anti-graft campaign, a top Chinese official said.
"(Xi) addressed the cases of Bo Xilai, Zhou Yongkang, Ling Jihua, Xu Caihou, Guo Boxiong and Sun Zhengcai. They had high positions and great power in the party, but they were hugely corrupt and plotted to usurp the party's leadership and seize state power," Liu said,
Sun had been expelled from the party and handed over to judicial authorities for further investigation. The announcement came two months after his downfall but no details of the investigation were released, according to the reports.
Bo who once regarded as rival to Xi five years ago is currently languishing in prison serving a life sentence.
Thousands of officials of the CPC at various levels, including the military have been punished in massive anti- graft campaign by Xi after he took over power in 2015.