BEIJING (REUTERS) - Wang Yang, one of China's best-known reformers who was passed over for promotion last year, is set to become one of four vice premiers, sources said, a role that will see his credentials for change tested to the full.

The largely rubber-stamp National People's Congress, or parliament, will name the four vice premiers and cabinet ministers in a tightly scripted ceremony at the Great Hall of the People in Beijing on Saturday.

Mr Wang made his name as the party boss of the export powerhouse of Guangdong province in south China, where he pushed for social and political reform.

Despite later toning down his reformist agenda, he missed out on a seat on the seven-member Politburo Standing Committee, the apex of power in China, at the party congress in November amid concerns of elders that he was too liberal.

At 58, Mr Wang will be the youngest of the vice premiers, who assist Premier Li Keqiang run the world's second biggest economy, four independent sources said.

Mr Wang will be the third-ranked vice premier.

Mr Zhang Gaoli, 66, who eked into the standing committee, will become No. 1 vice premier and help oversee the financial sector, said the sources, two of whom have ties to the leadership.

Ms Liu Yandong, 67, will be the second-ranked vice premier and be responsible for culture, education, health and sports, the sources said, requesting anonymity to avoid repercussions for discussing secretive elite politics with foreign media.

Mr Ma Kai, 66, one-time state planner, will be the fourth- ranked vice premier, the sources said.

Ms Liu, Mr Wang and Mr Ma are members of the party's 25-member Politburo, one notch below the standing committee.

The sources were divided over whether the country's agriculture ministry, which reports to a vice premier, will be overseen by Mr Wang or Mr Ma. The other portfolios up for grabs will be industry, transport, production safety, state assets, telecommunications and railways.

Whoever gets the agriculture portfolio will have to try to steer through policies aimed at transforming China's sprawling agricultural sector and improving long-term food security.

China plans to spend 40 trillion yuan (S$8 trillion) to bring 400 million people to cities over the next decade to turn the country into a wealthy world power with economic growth generated by affluent urban consumers.

"Li Keqiang is pushing for urbanisation. The countryside will be the main battlefield," one source with leadership ties told Reuters.

China needs to bring in the commercial and technological wherewithal that will allow its farms to feed an increasingly rich and urban population, even in the face of declining water supplies, land and rural labour.

But the vice premier overseeing agriculture will also need to ensure that the central government treads carefully, encouraging the transfer of land and the creation of advanced agribusinesses while addressing a widening urban-rural wage gap and protecting the interests of millions of small, subsistence farmers throughout the country.

Mr Wang has experience dealing with rural protests against land grabs, winning praise for deftly handling a crisis in the coastal village of Wukan in Guangdong in 2011, but observers have said his instinct for reform has usually been frustrated.

Mr Zheng Fengtian, a professor with the school of agricultural economics and rural development at Renmin University in Beijing, said the leader responsible for agriculture would likely have his hands tied.

"I think the new leader will continue the current agricultural policy mapped out by the (party's) previous Central Committee and the State Council (cabinet), which have clearly outlined ideas on agriculture development," he said.

"The key issue is urbanisation and I hardly expect any breakthrough beyond what has already been talked about."

Saturday's parliamentary session will also decide who will take over a number of key government ministries.

The sources said two front runners have emerged to take over as chairman of China's powerful state planner, the National Development and Reform Commission, including current vice-chairman Xie Zhenhua, also the country's chief climate change official, and Xu Shaoshi, the minister of land and resources.

Current vice-minister Pan Yue remains the main candidate to take over as minister of environmental protection, but incumbent Zhou Shengxian might stay on, the sources said.