Piling on the pounds increases the chances of breast cancer returning after treatment, a study has shown.

Women who put on a lot of weight after diagnosis were 14% more likely to suffer a potentially fatal recurrence of the disease, US scientists discovered.

The association was only seen in women who experienced "large weight gain", defined as a weight increase of 10% or more.

It did not apply to women who gained a "moderate" amount of weight, within 5% of what they tipped the scales at before diagnosis.

The trend applied both to women who started off heavy and those who were not overweight before getting cancer.

In fact women who were leaner to begin with were most at risk. A normal-weight woman who gained 10% or more after diagnosis had a 25% higher risk of dying from her disease than one whose weight remained stable.

"Most women are not gaining a large amount of weight following breast cancer diagnosis," said lead researcher Dr Bette Caan, from the Kaiser Permanente Division of Research in Oakland, California. "However, our analysis showed an association with poorer outcomes overall for those who do."

The findings were presented today at the annual meeting of the American Association for Cancer Research in Orlando, Florida.

Dr Caan stressed that moderate weight gain did not affect breast cancer prognosis.

"Women tend to worry about gaining weight after a breast cancer diagnosis," she said. "But it's actually only the larger weight gains that increase the risk of poor outcomes."

The study drew on data from the After Breast Cancer Pooling Project which looked at four groups of 18,336 breast cancer survivors.

Large weight gain occurred in 16% of the women overall.