China should phase out its unpopular one-child policy and let families have two children by 2015, an influential think-tank with close links to the government has proposed.

The suggestion comes from the China Development Research Foundation (CDRF), adding to growing calls for a relaxation of a policy first introduced over 30 years ago.

It would see the world's most populous country eventually abandon a measure that has been blamed for creating a demographic timebomb, with an ageing population foreshadowing huge economic and social problems.

The policy has also fostered a gender imbalance.

"Problems in population structure, quality and distribution have become increasingly visible and will have a profound impact on China's future social and economic development," the CDRF said in a report, according to a Xinhua news agency dispatch late Tuesday.

"China has paid a huge political and social cost for the policy, as it has resulted in social conflict, high administrative costs and led indirectly to a long-term gender imbalance at birth."

The report calls for family planning to be "loosened" in those provinces with stricter controls -- which are commonly urban areas -- allowing families nationwide to have two children instead of one by 2015.

All restrictions should be lifted by 2020 "as people will make more rational decisions on birth issues", the CDRF said.

The southern province of Guangdong asked Beijing in July for permission to relax the policy and allow couples where just one parent is an only child to have a second baby.

But the plan was dropped when Zhang Feng, director of the province's population and family planning commission, said there would be "no major adjustments to the family planning policy within five years".

Family planning officials in Beijing have defended the one-child policy in the past, claiming China's population -- currently 1.3 billion -- would have hit 1.7 billion without it.