In less than 25 years, Mexico City will be the world's largest city, with nearly 32 million people - two and a half times the projected population of London in the year 2000.

Sao Paulo will be close behind with a population of 26 million - triple the current size of Paris.

According to a recent report from the Population Reference Bureau, a Washington-based organization that studies world population and migration trends, cities in underdeveloped countries are growing so quickly that, by the end of this century, more than two-thirds of the world's total urban population will live in them.

For the first time in history, the report says, the largest Third World cities will be bigger than any of their counterparts in developed nations. If current population trends continue for the next quarter century, cities like Bogota, Manila, Karachi and Jakarta will triple in size. Lagos, with a current population of 2.1 million, will quadruple. Some cities, with populations already over 10 years. London and Tokyo, in contrast, will grow less than 1 per cent.

Problem resulting from such rapid growth, the study says, will be "gargantuan."

Nearly all of the new urban population, half of which will be rural emigrants and half native-born, will be poor. Slums and illegal shantytowns that surround Third World cities will balloon; transportation system that are already nighmarish will get worse; the number of unskilled unemployed will multiply.

While many of the problems are the unavoidable results of rapid development, population growth and scarce resources, they have been exacerbated, the study says, by poor administration and policymakers whose "attention . . . is elsewhere."

In a vicious in which the rich inevitably get richer and the poor get nowhere, the study outlines land-use policies andland speculation by the wealthy that provide nowhere for the poor to live. Forced to set up house-keeping in "illegal shanties on the city periphery, the poor are not provided with the most basic public services.

"Those with political and economic clout are first in line" for services like running water and heat, the study notes. "Public funds are invested in roads for the cars of the rich and universitites for their offspring."

Government efforts to industrialize and reduce imports have also resulted in capital-intensive, automated, large-scale industries that provide jobs for few.

Those jobs that do exist are often far out of range of the poor majorities, the report says, because of "rationing of public sector jobs, irrelevant credential requirements, excessive licensing controls, excessive educational prerequistes," and restrictive unions.

The report suggests some wide-ranging solutions, but points out that governments in such places have shown a marked reluctance to implement "long-run" rather than "short-run" alternatives.

For most cities, public housing projects are not the answer. Instead, the report suggest simply laying out housing tracts, alloting parcels, erecting simple shelters and paying some inner-city roads as inexpensive preliminary steps.

Transportation costs from the slums to inner city jobs are often prohibitive and, in Lima, for example, slum dwellers pay 10 times as much for water carried in on provate trucks as the more affluent pay for public plumbing.

In societies that are still primarily rural, like many in Africa, but whose urban centers are growing at alarming rates, the report recommends increased investment in agriculture to soften rural/urban income discrepancies and keep people on the farms. In Latin American, where agriculture ha s long been considered of lower priority than industry, it is too late for such action. The thrust must be toward housing, employing and educating those already in the cities.

The well-being of the cities, the report says, "will depend on the extent to which governments are prepared to recognize and take positive measures to accommodate this growth. All too often the response is either to ignore the problems of the future or hope that somehow they will disappear. In either case the result is neglect, a neglect that if continued will threaten the social fabric of many societies in the not-too-distant future."