JOHANNESBURG, South Africa — The United Nations’s top AIDS scientists plan to acknowledge this week that they have long overestimated both the size and the course of the epidemic, which they now believe has been ebbing for nearly a decade, according to U.N. documents prepared for the announcement.

AIDS remains a devastating public health crisis in the most heavily affected areas of sub-Saharan Africa.

But the far-reaching revisions amount to at least a partial acknowledgment of criticisms long leveled by outside researchers who disputed the U.N. portrayal of an ever-expanding global epidemic.

The latest estimates, due to be released publicly tomorrow, put the number of annual new HIV infections at 2.5 million, a cut of more than 40% from last year’s estimate, documents show. The worldwide total of people infected with HIV — estimated a year ago at nearly 40 million and rising — now will be reported as 33 million.

Having millions fewer people with a lethal contagious disease is good news.

Some researchers, however, contend that persistent overestimates in the widely quoted U.N. reports have long skewed funding decisions and obscured potential lessons about how to slow the spread of HIV. Critics have also said that U.N. officials overstated the extent of the epidemic to help gather political and financial support for combating AIDS.

“There was a tendency toward alarmism, and that fit perhaps a certain fund-raising agenda,” said Helen Epstein, author of “The Invisible Cure: Africa, the West, and the Fight Against AIDS.” “I hope these new numbers will help refocus the response in a more pragmatic way.”