South Koreans are likely to have the highest life expectancy in the world by 2030 and the United States one of the lowest among developed countries, the study showed.

"The fact that we will continue to live longer means we need to think about strengthening the health and social care systems to support an ageing population with multiple health needs," said Majid Ezzati, the lead researcher and a professor at Imperial College London's school of public health.

Led by Imperial scientists in collaboration with the World Health Organization, the study found that among high-income countries, the United States is likely to have the lowest life expectancy in 2030, with men and women expecting to live 79.5 and 83.3 years respectively - similar to middle-income countries like Croatia and Mexico.

This was partly due to a lack of universal healthcare in the United States, and also due to factors such as relatively high child and maternal mortality rates, and high rates of homicides and obesity, the study said.

In Europe, French women and Swiss men were predicted to have the highest life expectancies, averaging 88.6 years for French women and nearly 84 years for Swiss men.

South Korea came out top of the predictions, with the researchers predicting a girl born in South Korea in 2030 should expect to live 90.8 years, while a boy could reach 84.1 years.

"Many people used to believe that 90 years is the upper limit for life expectancy, but this research suggests we will break the 90-year-barrier," Ezzati said.

"We repeatedly hear that improvements in human longevity are about to come to an end.. (but) I don't believe we're anywhere near the upper limit of life expectancy - if there even is one."

The study, published in The Lancet medical journal on Wednesday, covered 35 developed and emerging countries, including the United States, Canada, Britain, Germany, Australia, Poland, Mexico and the Czech Republic.

South Korea's much greater average life expectancy would be due to several factors including good childhood nutrition, low blood pressure, low levels of smoking and good access to healthcare, new medical knowledge and technologies, the researchers said.