China's population may be smaller than the official 1.37 billion figure and closer to around 1.29 billion people - less than India's population - according to an independent Chinese demographer.

"China's real population may have been about 1.29 billion last year, 90 million fewer people than the official figure released by the National Bureau of Statistics," the South China Morning Post quoted Yi Fuxian, a scholar and demographer at the University of Wisconsin-Madison, as saying at a seminar in Peking University on Monday.

Yi, who has been a critic of the Chinese government's family planning measures and warned of an upcoming ageing crisis for China, said that the government had overstated China's population since 1990. He suggested the ageing crisis may be more severe than expected.

According to his numbers, India, with more than 1.3 billion people, is now the world's most populous country, overtaking China five years ahead of forecasts.

Yi said "there were 377.6 million new births from 1991 to 2016, less than the official figure of 464.8 million," the SCMP reported, noting that China's official figure was 1.37 billion.

Despite the population burdens faced by both countries, this landmark is being seen by most Chinese scholars as a warning rather than any cause for relief, with fears of a rapidly declining workforce and concerns of supporting the world's largest above 60 population.

Yi is far from alone in claiming China's official population data is overstated. "The government has overestimated the birth rate and underestimated the speed of demographic changes," Li Jianxin, another demographer who is at Peking University, was quoted as saying by SCMP. Another scholar, Liang Zhongtang of the Shanghai Academy of Social Sciences, also said official data wasn't accurate.

"The population data since the adaption of one-child policy has been seriously false and the family planning report has been overstated by 30 per cent," Liang was quoted as saying. "The birth control policy which has last nearly four decades is not in accordance with reality. It is imperative that the government should abandon the family planning system."

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Census 2011: India's population 1.21 billion; 17.5 per cent of world