Offering every woman diagnosed with breast cancer genetic screening would save hundreds of lives each year, a study suggests.
Currently, only those with risk factors such as a close family history of breast cancer are offered the check.
Researchers say screening everyone would enable women to make informed treatment choices and allow family members to take preventative action.
It would also be cost-effective for the NHS, they say.
In the UK, women diagnosed with ovarian cancer are offered testing to see if they carry a genetic mutation that would increase their risk of breast cancer.
But the same does not happen for breast cancer patients.
NHS England said it had committed to expanding genomic testing for patients with cancer as part of its long-term plan.
'My daughter has changed her destiny'
Alison Dagul found out she had ovarian cancer soon after being diagnosed with breast cancer.
But she did not find out she had inherited the Brca1 mutation, from her father, until after she had had treatment - and the knowledge would have changed her choice.
"If I'd known what I know now, I would have had a full mastectomy instead of a lumpectomy," she says.
"It would mean I'd need annual checks instead of having to be seen every 12 weeks.
"Everyone needs to be tested.
"If you're doing that, you are saving lives, just by giving people the knowledge.
But, she says, the knowledge means her daughter Gaby can make informed decisions about her health.
"She has already had a preventative double mastectomy, when she was 26, and she is now in the process of planning for surgery to prevent ovarian cancer," Alison says.
"It is a horrible cloud to live under but I am so grateful she has had the chance I didn't, to prevent herself from getting these cancers.
"It was heartbreaking finding out I'd passed it on to her.
"But now we have this knowledge - and she's changed her destiny."
'We should test more'
In the study, published in JAMA Oncology, UK researchers looked at data on 11,800 women diagnosed with breast cancer in the UK, US and Australia.
They then modelled what would happen if all breast-cancer patients were tested for the Brca1, Brca2 and Palb2 gene mutations, compared with the current approach.
Their analysis suggests one year's testing could save 2,102 cases of breast and ovarian cancer and 633 lives in the UK.
The National Institute for health and Care Excellence (NICE) advises the NHS on new tests or treatments using a measure called QALY - the cost of a year of good quality life. The upper limit is usually £20-30,000.
And the researchers in this study say offering genetic screening to all breast-cancer patients would cost £10,464 per QALY.
They took account of the cost of the genetic testing, cancer treatment, preventative surgery and screening or surgery for family members.
Ranjit Manchanda, a professor of gynaecological oncology at the Wolfson Institute of Preventative Medicine and Barts Cancer Institute, who led the study, said: "We should be testing more.
"This approach can ensure that more women can take preventative action to reduce their cancer risk or undertake regular screening."
Eve Appeal chief executive Athena Lamniosis said: "This study provides exciting evidence that offering screening to women at the point of diagnosis could save lives."
An NHS England spokesman said: "The new era of personalised medicine can bring life-changing benefits to patients which is why the NHS has committed to expanding genomic testing for patients with cancer as part of the Long Term Plan to transform care for millions of people across the country."