A drug for advanced ovarian cancer has been approved for use in newly diagnosed patients in England, after a trial showed it could delay progression of the disease for three years.
Up to 600 women with a hereditary type of the disease could benefit each year.
Health bosses said olaparib had the potential to make "a huge impact", giving a better chance of survival.
Most cases of ovarian cancer are diagnosed late, when there are few treatment options.
Olaparib is for women with advanced ovarian, fallopian-tube or peritoneal cancer who have a specific gene mutation - Brca - inherited from parents, which increases the risk of breast and ovarian cancer.
About five to 15 out of every 100 women with ovarian cancer have this faulty gene but there are concerns that less than a third of women with the disease in the UK are being tested for it.
Previously, olaparib has been available in the UK only to women who have already had at least three separate rounds of chemotherapy.
But now, patients in England who have responded to their first round of chemotherapy can also be given the tablets, which are taken twice a day.
Scotland is considering approving the drug to be used in the same way, while Wales will follow suit and Northern Ireland also likely.
'I didn't think I'd live to see my daughter's 21st'
Florence Wilks was diagnosed with advanced ovarian cancer in 2010, when she was 46, after experiencing symptoms such as extreme tiredness and backache for a couple of years.
She was given 12 to 18 months to live and had two major operations and four rounds of chemotherapy.
"You're told it's coming back, most likely," she says.
"How do you get your head round that? You think, 'This is horrific, life is going to be short.'"
Then, two years ago, Florence then started taking olaparib.
"Without this drug, I wouldn't be here," she says. "I'm so grateful for it."
"I didn't think I'd get to see my daughter's 21st or my son reach 18 - these are major milestones.
"Parenting is about supporting, advising and being there... and this last 28 months I've been able to do that.
"We spend a lot of time together because that time is so important."
Drugs advisory body NICE, the National Institute for Health and Care Excellence, said using olaparib at an earlier stage in treatment would bring the greatest benefit "and may have the potential to cure the disease".
In the trial, nearly 60% of patients receiving the drug did not see the disease get any worse after three years, compared with just 27% of patients taking a dummy drug.
But NICE said it was not yet clear if patients using it were living any longer, because those taking part in the trial had not been followed up for long enough.
Olaparib, made by AstraZeneca, is a pioneering type of cancer drug called a PARP inhibitor, which works by targeting an inherited genetic fault, causing cancer cells to die.
"Maintenance treatment with olaparib heralds a new era for women with ovarian cancer - this is the first time we have seen such dramatic improvements in progression-free survival," says Dr Susana Banerjee, consultant medical oncologist at the Royal Marsden, in London, who was involved in the trial of the drug.
"This means more women will have a longer time before relapse, time of chemotherapy and the possibility of increased survival."
Olaparib will be available immediately in England for this purpose, paid for through the Cancer Drugs Fund.
Charity Ovarian Cancer Action said the news was "a huge advancement in how we treat the disease" - but too many women were missing out on genetic testing in order to access the drug, despite government guidelines.
"Address this inequality must be a government priority to ensure women do not miss out on life-saving treatment," chief executive Cary Wakefield said.