The growth of mega-regions and cities is also leading to unprecedented urban sprawl, new slums, unbalanced development and income inequalities, as more people move to satellite or dormitory cities, the report warns.

The world's mega-cities are merging to form vast “mega-regions” which may stretch hundreds of miles across countries and be home to more than 100 million people, a major UN report says.

The phenomenon of the “endless city” could be one of the most significant developments — and problems — in the way people live and economies grow in the next 50 years, says U.N.-Habitat, the agency for human settlements, which identifies the trend of developing mega-regions in its twice-yearly State of World Cities report.

The largest of these, says the report, which was launched on Monday at the World Urban Forum in Rio de Janeiro, is the Hong Kong-Shenhzen-Guangzhou region in China, home to about 120 million people. Other mega-regions have formed in Japan and Brazil and are developing in India, west Africa and elsewhere.

This trend helped the world pass a tipping point in the last year, with more than half the planet's people now living in cities. Urbanisation is now “unstoppable”. Anna Tibaijuka, the outgoing director of U.N.-Habitat said: “Just over half the world now lives in cities; by 2050, over 70 per cent of the world will be urban dwellers. By then, only 14 per cent of people in rich countries will live outside cities and 33 per cent in poor countries.” The growth of mega-regions and cities is also leading to unprecedented urban sprawl, new slums, unbalanced development and income inequalities, as more people move to satellite or dormitory cities, the report warns.

“Cities like Los Angeles grew 45 per cent in numbers between 1975-1990, but tripled their surface area in the same time. This sprawl is now increasingly happening in developing countries, as real estate developers promote the image of a ‘world-class lifestyle' outside the traditional city,” say the authors.

Urban sprawl, they say, is the symptom of a divided, dysfunctional city. “It is not only wasteful, it adds to transport costs, increases energy consumption, requires more resources and causes the loss of prime farmland.” The report's co-author, Eduardo Lopez Moreno, said: “The more unequal cities become, the higher the risk that economic disparities will result in social and political tension. The likelihood of urban unrest in unequal cities is high. Cities that are prospering the most are generally those that are reducing inequalities.” But the development of mega-regions is regarded as generally positive, Mr. Moreno said. “They [mega-regions rather than countries] are now driving wealth.” He added: “Research shows that the world's largest 40 mega-regions cover only a tiny fraction of the habitable surface of our planet and are home to fewer than 18 per cent of the world's population, [but] account for 66 per cent of all economic activity and 85 per cent of technological and scientific innovation. The top 25 cities in the world account for more than half of the world's wealth, and the five largest cities in India and China account for 50 per cent of those countries' wealth.” Migration to cities, while making economic sense, is affecting the rural economy too. “Most of the wealth in rural areas already comes from people in urban areas sending money back,” Mr. Moreno said.

In a sample survey of world cities, the U.N. found the most unequal were in South Africa. Johannesburg was the least equal in the world, only marginally ahead of East London, Bloemfontein, and Pretoria.

Latin American, Asian and African cities were generally more equal, but mainly because they were uniformly poor, with a high level of slums and little sanitation. Some of the most egalitarian cities were found to be Dhaka and Chittagong in Bangladesh.

The U.S. is one of the most unequal societies, with cities like New York, Chicago and Washington less equal than Brazzaville in Congo-Brazzaville, Managua in Nicaragua and Davao City in the Philippines.

“The richest one per cent of [U.S.] households now earns more than 72 times the average income of the poorest 20 per cent of the population,” says the report. “In the ‘other America', poor black families are clustered in ghettoes lacking access to quality education, secure tenure, lucrative work and political power.” — © Guardian Newspapers Limited, 2010

More In: Comment | Opinion