China's sweeping overhaul of its anti-corruption architecture could enable President Xi Jinping to justify retaining his key ally and top graft buster Wang Qishan beyond retirement age, sources with ties to the leadership say.

Breaking the unwritten retirement rule could also set a precedent for Xi, 63, to defy current expectations that he will step down as party and military chief in 2022 and as state president the following year, the sources said.

A new National Supervisory Commission will combine the powers of several graft-fighting bodies, including Wang's Central Commission for Discipline Inspection (CCDI), the principal vehicle for Xi's signature anti-corruption drive.

That drive has punished more than a million party members, jailing top military figures and retired security tsar Zhou Yongkang, the most senior official toppled for corruption since 1949.

Wang is likely to head the new commission, the three sources said, a role that would under normal circumstances make a strong case for him to remain at Xi's side on the party's Politburo Standing Committee (PSC) - the top table of power in China.

For almost two decades, however, an informal age ceiling has meant only politicians aged 67 or younger have been eligible to remain on or be promoted to the PSC at the five-yearly party congress.

Wang turns 69 this year ahead of the 19th party congress this autumn.

"Wang Qishan is very likely to head the National Supervisory Commission," one of the sources told Reuters, requesting anonymity because he is not authorised to speak to media.

Whether he will stay on the PSC, and for a full term, will to some extent depend "on Wang's intent and health", the source said.

"If Wang Qishan doesn't retire, it makes it reasonable and lends legitimacy for Xi Jinping not stepping down in 2022," said a separate source with leadership ties.

George G. Chen, an expert on Chinese legal policy at the Berlin-based Mercator Institute for China Studies, said the formation of the new commission was a "clever, two-fold instrument", given criticism that the current anti-corruption campaign could be used as a political instrument.

"Xi Jinping could on one hand solve the problem of the legitimacy of the anti-corruption campaign, (while) also allowing Wang Qishan to (remain) in the Standing Committee," Chen told Reuters.

The CCDI and the State Council Information Office, which doubles as the spokesman's office for the government and Communist Party, did not respond to requests for comment.

The CCDI announced in January a plan to merge itself with the cabinet's Ministry of Supervision and the anti-corruption bureau of the top prosecutor’s office and corruption prevention bodies to create a super ministry. Pilot programmes have been set up in Beijing and the provinces of Zhejiang and Shanxi.

Currently, the CCDI can only investigate party members suspected of corruption. The Ministry of Supervision can interrogate civil servants, while the anti-corruption bureau of the prosecutor’s office is not empowered to detain suspects it is investigating.

The commission will eliminate duplication, with the power to supervise, question and detain all civil servants, party cadres, military personnel, judges and prosecutors, executives of state-owned enterprises, university staff and doctors and nurses of state hospitals who are suspected of corruption.

It will most likely be placed under the jurisdiction of the National People’s Congress (NPC), the country's rubberstamp parliament, but the sources said in reality the party leadership would have the final say.

Wang's current deputy, Li Shulei, 53, is the front-runner to eventually succeed Wang as head of the supervisory commission, and to secure a seat on the 25-member Politburo - one rung below the PSC - at the 19th congress, said the sources.

After a two-year stint as Fujian provincial propaganda chief, Li was the top graft buster in Beijing for a year before being named as Wang's deputy this January.

Since Xi declared war on corruption after taking office, 1.2 million of the party's 88 million members have been punished for violating discipline.