Rising global population will be held back by hundreds of millions of deaths through Aids, the UN predicts.
There is no vaccine for Aids
Estimates of rampant population growth are being revised down in the latest UN forecast because of the impact of the disease.
Three years ago the UN predicted the global population would grow from its current 6.3 billion to 9.3 billion in the year 2050.
But experts looking into the global Aids disaster now believe the spread of the disease makes that figure too high.
HIV/Aids is a disease of mass destruction and we do not see a vaccine coming soon
Instead, they believe the figure will be closer to 8.9 billion, a discrepancy of 400 million lives mainly due to the Aids pandemic.
The UN's World Population Prospects report estimates Aids deaths in the 53 worst affected countries as 46 million in the first decade of this century but says "that figure is projected to ascend to 278 million by 2050".
"The long-term impact of the epidemic remains dire," UN expert Joseph Chamie said.
"HIV/Aids is a disease of mass destruction and we do not see a vaccine coming soon."
AIDS IN AFRICA
Africa's growing epidemic
And the true toll could be even worse, as the UN forecast makes assumptions about HIV infection rates tailing off after 2010.
The experts believe education campaigns will start to have a global impact by this point.
But the latest data on the epidemic itself from UNAids shows that in some parts of the world, infection rates are rising rapidly.
And some experts have suggested that the benefits of public health campaigns may not always be permanent, pointing to rising levels of infection in the UK.
As reported recently, the prospect of an effective vaccine remains elusive with a round of tests generally regarded as a failure.
As well as the threat of Aids, there is also the effect of falling fertility levels in the Third World dropping beneath the rate needed to ensure long-term replacement of the population.
The UN said "future fertility levels in most developing countries will likely fall below 2.1 children per woman... at some time in the 21st century".
Mr Chamie added: "The world population will be significantly larger, substantially older and more concentrated in the cities of the Third World."
The report suggested by 2050, India will have replaced China as the world's biggest population with 1.531 billion people, compared with 1.275 billion in 2000.