Two Chinese academics have proposed a controversial idea to encourage childbirth as their country faces an ageing population: Make people childless people or parents of just one child pay into a "maternity fund".

The suggestion sparked a furious social media debate in a country whose people faced drastic family planning policies under the Communist Party, which enforced a one-child policy for decades.

The world's most populous country is now seeking to rejuvenate its greying population as concerns mount that an ageing and shrinking workforce could slow down its economy, while gender imbalances could lead to social problems.

Beijing loosened the rules in 2016, allowing people to have two children, but childbirths have not increased as much as forecast and there has been speculation the government will further ease restrictions.

The two academics made their suggestion in Tuesday's edition of the state-run Xinhua Daily, calling for those below the age of 40 and with fewer than two children should contribute annually to a fund that would offset childbirth costs for others.