LONDON (AP) - Women infected with the common sexually transmitted human papilloma virus could be more likely to develop cervical cancer if they have taken birth control pills for a long time, new research has found.

Some gynecologists believe that contraceptive pills might increase the risk of cervical cancer, but others are doubtful because previous studies have not ruled out the possibility that pill users might simply be more likely to be infected with the human papilloma virus, the main cause of cervical cancer.

Experts say the new study, conducted by the International Agency for Research on Cancer, an arm of the World Health Organization, lends support to the view that there is a real connection between birth control pills and cervical cancer.

However, Dr. Jack Cuzick, head of mathematics, statistics and epidemiology at Cancer Research UK in London, who was not connected with the study, said the findings should be interpreted cautiously because the women were only tested for the human papilloma virus, or HPV, once.

"Ideally, they should be positive on two occasions, at least six months apart, before you call them positive," he said.

Virtually all cervical cancer is caused by the human papilloma virus, or HPV.

Nearly all sexually active women will be infected by HPV sometime during their lives, but in most cases the immune system quickly eliminates it.

The key issue is why, in some cases, the virus is not eliminated. If the infection persists, the chances of cancer increase enormously.

"This study suggests that if you've got an HPV infection, oral contraceptives may actually be promoting the rate at which that progresses to cancer," Cuzick said.

The research was to be published Wednesday on the Web site of The Lancet medical journal.

The WHO researchers pooled evidence collected in eight earlier studies of 3,769 women from four continents, who were tested for HPV and told the investigators about their use of birth control pills.

A total of 1,853 had cervical cancer, while 1,916 did not. As expected, nearly all the women with cancer tested positive for HPV, while hardly any who were cancer-free had the virus.

The WHO researchers found that women infected with HPV who had used birth control pills for an accumulated total of five years or more were nearly three times more likely to develop cervical cancer than HPV infected women who had never taken the pill.

The increased risk persisted for up to 14 years after stopping the contraceptives.

Women who had taken the pill for 10 years or more were four times more likely to get the disease than those who had never taken it.

Using the pill for less than five years did not result in a higher chance of cervical cancer.

The study also found that women who had taken the pill were no more likely than the others to be carriers of HPV.

That indicates that rather than enhancing the chances of getting an HPV infection or of prolonging one - as some have suspected - the pill promotes progression of the infection to cancer, said David Skegg, head of the preventive and social medicine department at the University of Otago in New Zealand who was not involved in the research.

Women have about a 1 percent chance of developing cervical cancer. That means that, based on the study's findings, taking the pill for five years or more could push that chance up to about 3 percent and taking it for a total of 10 years could raise it to about 4 percent.

Worldwide, almost 360,000 women were diagnosed with cervical cancer in 1990, the latest date for which figures are available. Of those, 190,000 died of the disease. It is the second most common cancer in women.

Cervical cancer strikes 12,900 American women each year and kills 4,400 of them.

Copyright 2002 The Associated Press. All rights reserved.