BEIJING, Oct. 22 (Xinhua) -- Beijing's booming economy is drawing in economic migrants who look set to boost the population figures of the Chinese capital by almost five million over the next 12 years, according to a study released on Monday.

The report, called "Forecasting the Population of Beijing in the Future", predicts that the capital's population will soar to 20 million by 2020.

Although the natural birth rate of the city has seen zero growth for the last 5 years, Beijing's population has expanded because of the numbers of incoming residents, including the newly permanent residents who have been granted "hukou", or permanent residence certificates, and the migrants, according to the report, released by Institute of Population Research of Peking University.

Chen Gong, associate professor of the institute and one of the co-authors of the report, suggested that the total fertility rate of women in Beijing should be increased to 1.63, which means a woman should give birth to more than one child on average during her lifetime.

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.

The report also predicted that the city's aged population would see a 12 percent hike in the next 20 years, bringing more challenging demographic social problems for the local government.

Growth in the capital's population figures in the past five years has been phenomenal. The population has increased by over 10million, making it among the world's most populous cities.