BEIJING, Dec 4 (Reuters) - The population of China, already the world's most populous country despite its "one-child" policy, will grow to 1.5 billion people by 2033, with birth rates set to soar over the next five years, media said on Tuesday.

China's population, now 1.3 billion, would reach 1.45 to 1.46 billion by 2020, before reaching a peak of 1.5 billion in 2033, the Beijing News said, citing Zhai Zhenwu, head of a population research institute at People's University in Beijing.

"Currently, China is adding 16 million people a year. By 2012, annual (population increase) will reach a peak of 19 million," the Beijing News quoted Zhai as saying.

China's population was increasing at a "normal" rate, Zhai said, and needed to maintain the current birth rate to ensure it would not increase too quickly or slowly.

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.

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.

Officials have also cautioned that population controls are being unravelled by the increased mobility of China's 150 million-odd migrant workers, who travel from poor rural areas to work in more affluent eastern cities.

The influx of migrant workers into the capital had seen Beijing's population reach 17.4 million, the paper said in a separate report, only 600,000 short of a ceiling of 18 million the city government had set for 2020.

China has vowed to slap heavier fines on wealthy citizens who flout family planning laws, in response to the emergence of an upper class willing to pay standard fines to have more children.

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