The study suggests that by losing at least 10 pounds, between age 18 and 30, reduces the risk of cancer by up to 65%.

But it also showed that by gaining 10 pounds the women's risk of developing cancer before the age of 40 increased.

According to experts, the Breast Cancer Research study, is the first to link pre-menopausal weight gain and cancer.

It is already known that being overweight after the menopause increases women's risk of developing the disease.

The research team, from Canada, the U.S. and Poland, studied more than 2,000 women who were carrying the faulty BRCA1 or BRCA2 breast cancer genes.

The women were tested for BRCA 1 and 2 and questioned about their weight at ages 18, 30 and 40, and the team found that those women carrying the BRCA1 gene who lost weight saw the greatest benefit.

Maintaining a healthy weight is also one of the best ways for everyone to reduce their chances of getting cancer.

The women at 18, had an average weight of 142.5 pounds, and by the age of 30 they had lost an average of 18.6 pounds, ranging from 10 to 86 pounds.

It also appeared that slimming reduced the risk for women with another defective breast cancer gene, BRCA 2, but not to a significant degree.

However gaining weight heightened the risk of breast cancer for BRCA 1 mutation carriers who had borne at least two children, increasing their chances of being diagnosed before the age of 40 by 44% if they gained 10 pounds between the ages of 18 and 30.

The researchers suggest that carrying extra fat around the middle of the body could affect ovarian hormone and glucose metabolism, and promote insulin resistance, which are all factors linked to increased breast cancer risk.

Dr Steven Narod of the University of Toronto in Canada, who led the study, says the results suggest that weight loss in early adult life, protects against early-onset BRCA-associated breast cancers, and weight gain should also be avoided, particularly among BRCA 1 mutation carriers who elect to have at least two pregnancies.

Dr Kat Arney, cancer information officer at Cancer Research UK, says the study highlights the importance for women who have inherited an increased risk of breast cancer due to faults in their BRCA1 or BRCA2 genes,of maintaining a healthy bodyweight throughout adult life.

She adds that the results are interesting because previous studies have not found a link between pre-menopausal weight gain and increased cancer risk in the general population.

Arney says that maintaining a healthy weight is also one of the best ways for everyone to reduce their chances of getting cancer, but in particular, being overweight after the menopause significantly increases a woman's risk of womb and breast cancer.

Dr Sarah Rawlings, head of policy at the charity Breakthrough Breast Cancer, says the link between weight and breast cancer is complex and it is recommended that all women maintain a healthy weight throughout their lives by eating a balanced diet and exercising regularly.

Rawlings adds that women who are concerned about their weight or wish to lose weight should only do so in consultation with their GP.

The study is published in Breast Cancer Research.