Some women who believe they are at risk of breast cancer are having surgery to remove breasts rather than wait for genetic test results, a charity says.
Some patients are waiting up to two years for results, it is claimed
Breakthrough Breast Cancer says some women have had to wait for two years for their test results, despite a government target of eight weeks.
Genetic testing can find a problem in advance, allowing patients to receive preventative treatment.
Ministers insist the NHS is making progress in providing quicker results.
Every year in Britain about 2,000 women develop breast cancer because of flaws in their genes - roughly 5% of the total.
The Department of Health said up to £18m had been invested since 2003 to help modernise NHS genetic laboratories.
"The laboratories are now working very hard to get their new equipment and working practices up to speed in order to meet these standards," a department spokesman said.
"They are making excellent progress towards this important goal."
But Breakthrough Breast Cancer says the waiting time for test results has risen for the past three years.
It says on average they are taking six months, and some patients feel they cannot wait that long.
The charity's chief executive, Jeremy Hughes, said the lengthy wait was forcing people to "make decisions about what they can do to save their lives".
"If that means radical surgery, they will have that surgery."
Mother-of-two Oonagh Wilson, 37, is one patient who made what she described as the "extremely difficult" decision to have her breasts removed before getting her genetic test results.
Her mother was diagnosed with breast cancer in 2002 and died after submitting a blood sample for genetic testing as there had been a strong family history.
Ms Wilson said that waiting had been "emotionally draining" and felt like a "time bomb".
"It's a day-to-day battle to keep it at the back of your head," she said.
"If I didn't have the preventative surgery, and I had to wait for two or three years for the results to come, I don't think I would have been able to fulfil my life."