Women on hormone replacement therapy (HRT) are at higher risk of breast cancer, a study has found.

Ending HRT leads to a 28% drop in the risk of developing the disease within one year, the research shows.

It suggests patients who spend at least five years taking a combination of the hormones oestrogen and progestin double their risk of breast cancer every 12 months.

The study was a follow-up to the landmark US Women’s Health Initiative (WHI) trial which in 2002 established links between breast cancer, blood clots and strokes and combination HRT.

Because the results were so striking, the trial was halted three years early.

After publication of the WHI data, use of hormone therapy plummeted in the US from 60 million prescriptions in 2001 to 20 million in 2005.Breast cancer rates also declined significantly, suggesting a strong link between HRT and cancer risk.

But some experts still questioned the connection, saying the dip in breast cancer rates may have been related to mammogram screening.

Professor Marcia Stefanick, from Stanford University in California, who co-authored the new study, said the latest results should settle the debate.

"This is very strong evidence that oestrogen plus progestin causes breast cancer," she said.

"You start women on hormones and within five years their risk for breast cancer is clearly elevated.

"You stop the hormones and within one year, their risk is essentially back to normal. It’s reasonably convincing cause-and-effect."

Researchers looked at data from more than 15,000 women who took part in the original WHI trial, and more than 41,000 women enrolled in a separate observational study.

Results for the groups were quite similar.

The findings appear in the New England Journal of Medicine.

Via news.sky.com