• Race behind division in US cities, says UN report • Beijing is most egalitarian place in the world

Growing inequality in US cities could lead to widespread social unrest and increased mortality, says a new United Nations report on the urban environment.

In a survey of 120 major cities, New York was found to be the ninth most unequal in the world and Atlanta, New Orleans, Washington, and Miami had similar inequality levels to those of Nairobi, Kenya Abidjan and Ivory Coast. Many were above an internationally recognised acceptable "alert" line used to warn governments.

"High levels of inequality can lead to negative social, economic and political consequences that have a destabilising effect on societies," said the report. "[They] create social and political fractures that can develop into social unrest and insecurity."

According to the annual State of the World's cities report from UN-Habitat, race is one of the most important factors determining levels of inequality in the US and Canada.

"In western New York state nearly 40% of the black, Hispanic and mixed-race households earned less than $15,000 compared with 15% of white households. The life expectancy of African-Americans in the US is about the same as that of people living in China and some states of India, despite the fact that the US is far richer than the other two countries," it said.

Disparities of wealth were measured on the "Gini co-efficient", an internationally recognised measure usually only applied to the wealth of countries. The higher the level, the more wealth is concentrated in the hands of fewer people.

"It is clear that social tension comes from inequality. The trickle down theory [that wealth starts with the rich] has not delivered. Inequality is not good for anybody," said Anna Tibaijuka, head of UN-Habitat, in London yesterday.

The report found that India was becoming more unequal as a direct result of economic liberalisation and globalisation, and that the most unequal cities were in South Africa and Namibia and Latin America. "The cumulative effect of unequal distribution [of wealth] has been a deep and lasting division between rich and poor. Trade liberalisation did not bring about the expected benefits."

The report suggested that Beijing was now the most egalitarian city in the world, just ahead of cities such as Jakarta in Indonesia and Dire Dawa in Ethiopia.

In Europe, which was generally more egalitarian than other continents, Denmark, Finland, the Netherlands and Slovenia were classed as the most equal countries with Greece, the UK and Spain among the least. "Disparities are particularly significant in the cities of eastern Europe, larger Spanish cities and in the north of England," it said.

It documents the seemingly unstoppable move of people away from rural to urban areas. This year it is believed that the number of people living in urban areas exceeded those in the countryside for the first time ever, but the report says there is no sign of the trend slowing.

"The dramatic transition between rural and urban communities is not over. Urbanisation levels will rise dramatically in the next 40 years to reach 70% by 2050," it predicts.

The most dramatic urbanisation has been taking place in China, with many millions of people moving from the countryside to cities. The report says 49 new cities have been built in the past 18 years. The rapid transition to an urban society has brought great wealth but also many negative results.

"China has attained some of the deepest disparities in the world with urban incomes three times those in rural areas. Inequalities are growing, with disproportionate rewards for the most skilled workers ... and serious problems for the unemployed and informal workers."

Urban growth rates are highest in the developing world, which absorbs an average 5 million new urban residents a month and is responsible for 95% of world urban growth. The report predicts that Asian cities will grow the most in the next 40 years and could have 63% of the world urban population by 2050.

Tokyo is expected to remain the world's largest mega city, with 36.4m people by 2025. But Mexico City, New York, and Sao Paulo could give way in the league table to Mumbai, Delhi and Dhaka. Kinshasa and Lagos are the two African cities expected to grow the most, with each adding more than 6 million people by 2025.

Rather than countryside to city movement, which has marked rapid population growth in the last 40 years, the UN expects more people to move from city to city.

Capital cities in particular are attracting much more of countries' investments and are growing fast. Some are becoming home to nearly half a country's population.

But the report also identified what it believes is the emergence of a new urban trend, with many cities now shrinking in size. The populations of 46 countries, including Germany, Italy, Japan and most former soviet states, are expected to be smaller in 2050 than they are now, and in the past 30 years, says the report, more cities in the developed world have shrunk than grown.

It found that 49 cities in the UK, including Liverpool and other old industrial centres in the north of England, and 100 in Russia reduced in size between 1990 and 2000, mainly because of unemployment. In the US 39 cities are smaller now than they were 10 years ago.

The reasons for the decline of cities was mostly economic, but the report says that the environment is now an important factor.

Air quality and pollution from mines, power plants and oil exploration have been responsible for population losses in India, Mexico and Africa, it says. "Cities tend to struggle most with health-threatening environmental issues, such as the lack of safe water, sanitation and waste."