Two-thirds of people in the world will be living in cities by 2050 and the boom will be concentrated in India, China and Nigeria, according to United Nations estimates released on Wednesday.

The world’s rural population will peak in a few years then decline by 2050, according to the report by the UN’s population division.

Tokyo is currently the world’s largest city with 37 million people, followed by Delhi with 29 million, Shanghai with 26 million, and Mexico City and São Paulo, each with around 22 million inhabitants. Cairo, Mumbai, Beijing and Dhaka all have close to 20 million inhabitants.

However, Delhi will overtake Tokyo in top spot by around 2028, the report said.

At about the same time, India is expected to surpass China as the country with the world’s largest total population.

Around 55% of the world population lives in urban areas today, increasing to 68% by 2050. India, China and Nigeria will account for more than a third of that expansion.

There will also be more megacities. In 1990 there were just 10 megacities, classed as places with populations of 10 million or more. There are now 33 megacities and by 2030, 43 megacities are projected, mostly in developing countries.

However, a few cities in Japan and South Korea – for example, Nagasaki and Busan – have experienced population decline since 2000. Several cities in eastern Europe, such as in Poland, Romania, Russia and Ukraine, have lost population since the turn of the century.

However, urbanisation could be seen as positive, said John Wilmoth, director of the population division.

“The increasing concentration of people in cities provides a way of more economically providing services,” he said. “We find that urban populations have better access to health care and education.”

The concentration of population may also help to minimise our environmental impact on the planet, he said, and help cities design policies and practices to prepare for the influx.