Almost one billion people worldwide are living in slums according to a United Nations report published today.
The UN has warned without radical changes that number could double in 30 years.
By 2050, the UN says, there may be 3.5 billion slum-dwellers, out of a total urban population of about 6 billion.
The head of the UN human settlements programme UN-Habitat, Dr Anna Tibaijuka, said the persistence of slums should shame the entire world.
The programme has compiled what it says is the most comprehensive report ever on slums, based on studies of 37 cities across the globe.
The report says that in 2001, 924 million people - 31.6% of the world's urban population - lived in slums, most of them in developing countries.
It claims the number of slum-dwellers increased substantially during the 1990s and calls on governments to introduce policies to curb the problem.
Dr Tibaijuka said increased globalisation had contributed to the problem and needed to be regulated.