China, worried about an ageing population, is studying scrapping its controversial one-child policy but will not do away with family-planning policies altogether, a senior official said on Thursday.

With the world's biggest population straining scarce land, water and energy resources, China has enforced rules to restrict family size since the 1970s. Rules vary but usually limit families to one child, or two in the countryside.

"We want incrementally to have this change," Vice Minister of the National Population and Family Planning Commission Zhao Baige told reporters in Beijing.

"I cannot answer at what time or how, but this has become a big issue among decision makers," Zhao added. "The attitude is to do the studies, to consider it responsibly and to set it up systematically."

The average number of children that would be born to a woman over her lifetime has decreased to 1.8 in China today, from 5.8 in the 1970s, and below the replacement rate of 2.1.

China says its policies have prevented several hundred million births and boosted prosperity, but experts have warned of a looming social time-bomb from an ageing population and widening gender disparity stemming from a traditional preference for boys.

Still, the government has previously expressed concern that too many people are flouting the rules.

State media said in December that China's population would grow to 1.5 billion people by 2033, with birth rates set to soar over the next five years.

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