The number of British breast cancer patients over the age of 65 will have quadrupled in less than 30 years, with the number of sufferers reaching one million.
At present, 340,000 older women are currently living with the disease, but this will reach 1,220,000 by 2040, a study has found.
Researchers at Kings College London, funded by Macmillan Cancer Support, found that 79 per cent of women living with breast cancer will be over 65, compared to 59 per cent today.
These figures have raised concerns that the NHS is not prepared to cope with such a huge increase in breast cancer patients, and that older women with breast cancer are not being given adequate treatment.
Ciarán Devane, chief executive at Macmillan, said: "The NHS needs to take heed of these figures. It is already struggling to provide adequate care for older breast cancer patients. We need to change the way we care for older breast cancer patients now - so that we are prepared for such a dramatic increase in numbers."
Currently, around 48,000 women are diagnosed with breast cancer every year and 1,000 women die from the disease every month. Breast cancer accounts for almost a third of all cancers in women and one in eight women will develop breast cancer at some point in their lives.
The study found that older women are much less likely to undergo breast conserving surgery, and just a small percentage have breast reconstructive surgery. At present, older women are also much more likely to be diagnosed later with advanced breast cancer.
Devane said: "Older people must be provided with the right treatment at the right time at the correct level of intensity.
"We can never assume that because a woman is older that she will not cope with surgery or that she is less interested in body image than a younger woman. It is our duty to ensure that every cancer patient has access to the best possible care.
"We can no longer tolerate the present situation where too many cancer doctors are making assumptions based on age which often result in older women receiving inadequate care for their breast cancer."
Dr Rachel Greig, senior policy officer at Breakthrough Breast Cancer, said they are also concerned that women are being provided treatment based purely on their age.
"It is startling to think that we could have a million older women living with breast cancer within a generation. It shows that we have a rapidly ageing population - but also that we are getting much better at treating women with breast cancer," she said.
"However, increasingly we should think less about the age of patients, and more about their individual need. This will ensure the best standards of care for women living with the disease."