Sixty-eight percent of the world's population will live in urban areas by the year 2050, the United Nations has said, up from 55% at present.

The UN report predicted an extra 2.5 billion people living in ever-expanding cities in the next 30 years, with as much as 90% of the urban growth centred on Asia and Africa.

The boom in city-dwellers will be concentrated in certain countries, with 35% of urban expansion occurring in India, China and Nigeria alone between 2018 and 2050.

That will add up to an extra 416 people in towns and cities in India, 255 million in China and 189 million in Nigeria.

The number of people living in urban areas worldwide has risen rapidly from 751 million in 1950 to 4.2 billion in 2018.

Despite its lower rate of urbanisation, Asia contains 54% of all urban residents in the world, followed by Europe and Africa with 13% each.

The general migration from rural to urban areas, combined with overall population growth, will lead to the planet having 43 megacities by 2030, each home to more than 10 million people. There are currently 31 megacities.

Tokyo is currently the largest city on the planet with 37 million inhabitants, followed by New Delhi with 29 million, Shanghai with 26 million and Sao Paulo and Mexico City with 22 million apiece.

Cairo, Mumbai, Beijing and Dhaka are all closing in on the 20-million inhabitant mark.

The UN study, carried out by the world body's Population Division, predicts that Tokyo's population will start to decline in the coming two years and that it will be surpassed as the world's largest city by New Delhi towards 2028.

Population Division director John Wilmoth said: "When urban growth is rapid, insuring access to housing, water, sanitation, electricity, public transport, education and health care for all is especially challenging."

Speaking at a UN news conference, he said:"Managing urban growth to insure that it is sustainable has become one of the most important development challenges of the current century."

Overall, urbanisation can be seen as positive, he said.

"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 also may help minimise humans' environmental impact on the planet, he added.

"However, the challenge is that in many countries it's taking place so rapidly ... and large slum areas have developed that are not maybe achieving the goals of sustainable development the way that we would like."