"I knew I had the gene for breast cancer, BRCA1, and that it was only a matter of time, so why wait until the cancer arrived? Better to have it right away without the need for chemotherapy," she said.
It was the first preventative mastectomy of its kind in the UK, and five years on Mrs Watson, now 43 - the same age at which her mother died - has never felt better and there is no sign of the disease.
She is happy with her own health, but angry that in future women may be denied tests for the gene which would be one of the many covered by yesterday's European Parliament decision on genetic patenting, which will allow companies to claim fees and royalties for the use of tests which they have developed.
Myriad Genetics, of Salt Lake City in the United States, which has already applied to patent the BRCA1 gene in Europe, at present charges about $2,400 (pounds 1,472) for the test in the US.
"I have the gene and there is no doubt that if I had not had the mastectomy I would probably not be here today. My mum died at 43, my cousin at 38, and others at similar ages," said Mrs Watson who led a campaign against patenting of genetic tests and who a year ago launched the Hereditary Breast Cancer helpline.
She became aware of a likely genetic link when she was 16 and her mother died. Her grandmother had also had breast cancer twice, and had died of ovarian cancer, a malignancy in which the BRCA1 gene is also implicated. When she moved to the Peak District eight years ago, she found more family members with the disease.
"I didn't know much about genes at the time, but it was blindingly obvious to me that it was hereditary. I went to doctors to see what courses of action were open to me and they all talked about catching it early. It occurred to me that the only sure way was to rid of the at-risk tissue before the cancer came. I think everyone at the time thought I was being too drastic, even irrational, but I knew it wasn't drastic. My only objective was to stay alive and to me dying was the drastic bit, not what I was going to have done," Mrs Watson said.
"Everyone should have the right to have a genetic test and take whatever action is necessary to save their lives. By allowing companies to patent things like this and charge for them, we will in effect be denying some people the right to a cure."
Although no figures are available, it is estimated that at least 100 women a year in Britain now opt to have a preventative mastectomy because of extensive family history or as a result of positive genetic testing.
Britain's leading geneticists have warned the Government that gene- patenting will cost the NHS millions of pounds in royalties, mostly to American biotech companies.
But Bill Hockett, director of corporate affairs for Myriad Genetics, said: "The gene would probably not have been discovered but for the potential of patenting. Without the protection that the patent affords, a company could not invest hundreds of millions of dollars in getting it to the market place."
t Hereditary Breast Cancer helpline: 01629 813000.Reuse content