Beijing's population is expected to top 19.5 million by 2015 and climb to 21.4 million by 2020, according to a research report released on weekend.
The figure is much more than the city's target of 18 million for 2020, according to the Strategic Report on Population of Beijing, Tianjin and Hebei," released at the Beijing Population and Development Forum held in Beijing.
Qu Zhenwu, a professor of Institute of Population Research attached to Renmin University, attributed the rapid population growth in Beijing to continuous influx of migrants.
"Although Beijing saw zero growth in natural birth rate for the past five years, its population kept expanding as a result of a mass of migrants," he said.
The expert suggested that the population of Beijing be kept under 17 million while the total population in Beijing and its surrounding Tianjin Municipality and Hebei Province below 110 million.
Researchers called for closer cooperation in economic development in Beijing, Tianjin and Hebei to narrow gaps among the three regions.
Beijing had 15.6 million permanent residents at the end of last year, a rise of 1.4 percent, or 220,000 people, from 2005, according to the city's family planning committee.
Growth in the capital's population over the past five years has been rapid. The population has increased by over 10 million, making it among the world's most populous cities.