Between 20 million and 30 million people are moving each year to towns and cities in search of a better life. Most, however, will end up in slums and squatter camps, the hallmarks of the mega-city.
By 2000, only three of the largest cities - Tokyo, New York and Los Angeles - will be in the industrialised world, and the global population will have risen from 5.2 billion in 1990 to about 6.2 billion.
Current estimates suggest that an even higher projection of 6.4 billion is more likely, Diana Brown, chairman of Population Concern, said. 'Unfortunately, in recent years, we have been closer to following previous high projections rather than medium ones.'
The charity says that world population is increasing each year by well over 90 million people - equivalent to the combined population of the United Kingdom, Ireland, the Benelux countries, and Denmark. It estimates that by 2050, it could have risen to between 7.8 and 12.5 billion.
Ms Brown said: 'For the future of our planet and its inhabitants, we must hope that we do not follow the high projection.'
Although there have been encouraging signs that contraception and family planning is begining to have an effect on the rate at which some countries are growing, the number of children in the world today ensures the overall growth in population.
Ms Brown said that a third of the world's population will become old enough to have children during the Nineties.
'The only thing that rises faster than population is unemployment,' she said. 'There will be a dramatic rise in population, but how dramatic depends on what we do now.'
The growth of the mega-cities was inevitable, she said. 'People go to towns for the hope that the opportunities will present. But the real misery is the lack of infrastructure prepared for them.'
In many of Asia's largest cities, between a quarter and three-quarters of people live in slums or squatter settlements. By the end of the decade it is estimated that 60 per cent of the urban population of Asia will be living like this, according to Population Concern.
In Africa, which is experiencing the fastest population growth rate in the world, many cities will double in size within 12 years because of the high birth-rate and mass emigration from impoverished rural areas.
Ms Brown said there is now clear evidence that women in even the poorest countries want to limit the size of their families, and it is only lack of access to effective contraception and education that prevents them.
Many fertility surveys over the past few years show that women want fewer children than they have, she said. 'Apart from access to family planning services, it is the status of women in society that is important. When women have control over their own lives, they have fewer children.'
Population Concern is calling on the governments of the industrialised world to increase the amount of development aid dedicated to family planning services.
At a meeting of the United Nations Population Fund in 1989, the participating countries agreed to contribute an annual sum of dollars 9bn ( pounds 6bn) to family planning in an attempt to restrict population growth to the medium projection, Ms Brown said. 'Four years later, current spending is estimated to be about half that figure.'
Annual Report 1993, Population Concern, 231 Tottenham Court Road, London.
Leading article, page 13