WASHINGTON (Reuters) - Researchers looking into the long-term health effects of hormone replacement therapy said on Wednesday they had made the strongest case yet that the pills raise the risk of breast cancer.

But other experts and one company that makes hormone replacement therapy (HRT) pills said they still dispute the conclusion that a recent drop in US breast cancer cases means that HRT was responsible for the disease.

All the experts agree that HRT can be used by women suffering severe menopause symptoms, but they should use the lowest possible dose for the shortest period of time.

The findings stem from the Women's Health Initiative study, which found in 2002 that women taking HRT had higher rates of breast cancer, stroke and heart disease.

Women stopped taking the drugs wholesale and sales plummeted. A few years later, breast cancer rates dropped and some experts held this out as evidence that HRT caused breast cancer.

Dr. Rowan Chlebowski of the Los Angeles Biomedical Research Institute and other researchers re-analysed data that showed this and said the relationship is real. They also looked at data on 41,449 women enrolled in another Women's Health Initiative study.

Some had argued that a five percent drop in the rates of mammogram screening were in fact responsible for the drop in breast cancer diagnoses, but Chlebowski's team report in the New England Journal of Medicine that they find no evidence of this.

"The increased risk of breast cancer associated with the use of estrogen plus progestin declined markedly soon after discontinuation of combined hormone therapy and was unrelated to changes in frequency of mammography," they concluded.

"The difference in frequency of mammography use of two percent between 2002 and 2003 for women using hormones is insufficient to account for the 43 percent reduction in the incidence of breast cancer," they wrote.

Most of the women in the study had been taking Prempro, a combined estrogen-progesterone pill made by Wyeth.

"We don't believe the article supports the theory that the decline in use of estrogen plus progesterone caused a one-time abrupt nationwide decline in breast cancer incidence," Wyeth spokeswoman Gwendolyn Fisher said in a telephone interview.

"They don't offer an explanation of why breast cancer rates remain stable today when HRT rates continue to decline," she added.

The International Menopause Society agreed.he decline in breast cancer rates started at least three years before the Women's Health Initiative study was halted," the group said in a statement.