The world population will reach 11 billion by the end of the century - 800 million more people than previous estimates had forecast.
A report by the United Nations said the figure was adjusted because fertility rates in Africa had not fallen as quickly as anticipated.
The population of Africa is 1.1 billion and is predicted to quadruple to 4.2 billion by 2100.
Adrian Raftery, of the University of Washington, said: "The fertility decline in Africa has slowed down or stalled to a larger extent than we previously predicted, and as a result the African population will go up."
The UN used statistical models developed by Raftery and his colleagues that better predicted fertility. This was then combined with UN data to predict the long-term consequences of changing fertility patterns in Africa.
"These new findings show that we need to renew policies, such as increasing access to family planning and expanding education for girls, to address rapid population growth in Africa," Raftery said.
Researchers found that there will not be many other significant population changes across the world, with Europe expected to see a small decline because fertility falls below the replacement level. Other nations are expected to witness population increases because of longer life expectancies.
Eight of the top 10 countries poised to increase most are in Africa. India also features in the top 10 as does the US, where the population is expected to increase by 146 million, or 46%.
The global population reached seven billion in 2011, up by one billion in just 12 years.
Raftery said that there was no end in sight to the world's growing population. He said that the issue had dropped down the global agenda while more attention was focused on poverty and climate change - both phenomena that have a direct effect on population trends.
The UN report gave a high and a low range with 17 billion at the top end and seven billion at the bottom.
Rafferty's own range was much narrower - from nine billion to 13 billion in 2100.