The number of pensioners living with breast cancer is set to quadruple by 2040, researchers said.
In less than three decades there will be 1.2 million women aged over 65 living with the disease - almost four times today's figure of 340,000, according to researchers from King's College London.
At present, 59% of all breast cancer survivors are over the age of 65 but the research, funded by Macmillan Cancer Support, suggests that this figure will increase to 73% by 2040.
The data, published in the British Journal of Cancer, also suggests that by 2040 breast cancer will account for more than half of all cancer cases in women in the UK.
Ciaran Devane, chief executive of Macmillan Cancer Support, 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.
"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: "It is startling to think that we could have a million older women living with breast cancer within a generation."