The number of pensioners with cancer will treble by 2040, researchers suggest.
In three decades' time, 4.1 million over-65s will be living with the disease, compared with 1.3 million in 2010, the study finds.
Nearly one in four older people will receive a cancer diagnosis in 2040, almost double the proportion in 2010, the Macmillan Cancer Support-funded study suggests.
The sharp rise in the number of cases could be attributable to a number of causes such as an ageing population, increased incidence of cancer and increasing cancer survival rates, said the researchers at King's College London.
Professor Henrik Moller, one of the study authors, said: "The aim of this research is to provide long-term projections of cancer prevalence in the UK. The research shows that large increases can be expected in the oldest age groups in the coming decades and, with this, an increased demand upon health services."
Macmillan Cancer Support chief executive Ciaran Devane said: "The care of older cancer patients is the ticking time-bomb for society. These stark predictions should act as a warning to the NHS and social care providers of the problems ahead if older cancer patients are not offered the best treatment and support.
"We have a moral duty to give people the best chance of beating cancer, regardless of their age. For cancer survival to improve, older people must be given the right treatment at the correct level of intensity, together with the practical support to enable them to take it up.
"The barriers to older people getting treatment must be tackled. If we don't get this right now, many older people will be dying unnecessarily from cancer in the future."