The world's population, currently 5.4 billion, is likely to top the 10 billion mark by the middle of the next century, much sooner than previously thought, a United Nations report said Monday.

Experts are revising their estimates upward despite striking successes by family-planning programs in spreading the use of contraception and bringing down birthrates, said the annual report by the U.N. Population Fund (UNFPA)."Instead of a stable total of about 10.2 billion in 2085, the world may well reach 10 billion by about 2050, and significant growth will continue for another hundred years after that," said the report, "The State of World Population."

"Population may eventually level off at about 11.6 billion," it added.

The report, which gave no reason for the revised estimates, said that in the Third World the rapid population rise was straining health and education systems, hurting the environment, causing explosive urban growth and complicating food supply.

Developing countries' cereal imports, which stood at 69 million tons in 1983-85, were expected to total 112 million tons by the end of the century, added the report, which was published in London.

It said the population was growing fastest in Africa and would expand from 650 million today to 900 million by the year 2000, an annual 3 percent rise - "the highest regional growth rate the world has ever seen."

The population of Nigeria, Africa's most populous state with 108 million people, would double in the next 20 years, it said.

At the same time, UNFPA pointed to a dramatic increase in the use of modern contraceptive methods in developing countries, from 10 percent of couples in the 1960s to 51 percent today.

It said its target was to raise this to 59 percent by the end of the century.

"Special attention will be needed to develop better methods for men, to encourage them to take more more responsibility for family planning," UNFPA executive director Nafis Sadik said in an introduction.

Fertility - the number of births per woman - was also dropping in all parts of the world, with particularly sharp falls in Thailand, Indonesia and South Korea, the report said.

The report said the cost of meeting its family planning targets would be about $9 billion per year by the end of the century, double today's rate, and international aid would be needed to cover about half of this.

But it said this would be "far smaller than the cost of failure" measured in the extra education and health care that would be needed.