Former Prime Minister Chung Un-chan declared his backing for opposition presidential candidate Moon Jae-in on Tuesday, saying he believes he and the liberal contender can share his vision that bigger and smaller firms should seek shared growth.

Chung held the country's No. 2 post in 2009-2010 under President Lee Myung-bak before serving as head of the National Commission for Corporate Partnership until earlier this year, a post that promotes "shared growth" between conglomerates and smaller businesses.

The decision was unexpected as Chung served as prime minister under the current conservative government.

"After reading candidate Moon's campaign pledges and meeting him in person, I determined that he can share the value of shared growth with me," Chung said in a statement.

Chung said he has traveled around the country in the last several months to help promote shared growth, but he felt a limit to what he can do alone and wanted to cooperate with those sharing his shared growth vision and willing to promote the value.

"There is no future for small- and medium-sized enterprises with creativity and passion in the current South Korean economy over which chaebol cast thick clouds," Chung said, emphasizing the massive influence of big conglomerates. "We need to make the economic ecosystem a virtuous circle and I agreed with Moon that the only way toward it is shared growth."

Chung, an economics scholar and professor emeritus of Seoul National University, made up his mind to support Moon of the main opposition Democratic United Party after meeting him in person on Sunday and again Tuesday morning, sources said.

Ahead of the 2007 presidential election, Chung had once been considered a potential opposition presidential candidate, but he later joined the Lee administration before leaving the job after the parliament rejected his proposal to revise a project to build an administrative city in central South Korea. (Yonhap News)