The number of older people living with cancer will more than treble by 2040, say experts, who warn of a "ticking timebomb" for society if the NHS and social care systems are not geared up to help them in time.

Macmillan Cancer Support says the number of over-65s who have received a cancer diagnosis will go up from 1.3 million in 2010 to 4.1 million in 2040.

Its estimates – based on research from Kings College London, which it funded – mean that nearly a quarter of all older people (23%) will be living with cancer within 30 years. At the moment, the figure is 13%.

The biggest increase, according to the study – published online by the British Journal of Cancer – will be in lung cancer in women, which is expected to more than double from 319 to 813 per 100,000 people. While the number of men smoking has declined markedly since the 1970s, women have not quit in the same numbers.

Professor Henrik Møller, one of the study's authors at King's College London, 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," he said.

Cancer is predominantly a disease of older age. More than 60% of people living with cancer are over the age of 65 and half are over 70. The numbers of older people with cancer are rising sharply because the population is living longer, but also because cancer rates are increasing and treatments are improving so that more patients are surviving for longer or are cured altogether.

But in spite of this, Macmillan argues that older people who are diagnosed do not always get the best care. Decisions about which treatments to give people are too often based on age, rather than on fitness, says the charity.

Older patients are less likely to have radical surgery or radiotherapy and some studies suggest that their survival is less likely because of it. Older people are also more likely to find it harder to recover after their treatment and more likely to feel abandoned by the health service than younger people.

Ciarán Devane, chief executive of Macmillan Cancer Support, said: "The care of older cancer patients is the ticking timebomb 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," he said.

A Department of Health spokesperson said: "It's good news that improvements in cancer treatment mean more people are surviving after cancer.

"We know more can be done to improve cancer care for older people, which is why we are working with Macmillan Cancer Support and Age UK on a £1m programme to ensure that older people's needs are properly assessed and met.

"From 1 October 2012, it will be unlawful to discriminate in health and social care on the basis of age. Adults of all ages will benefit from better access to services, and for the first time people will have a legal right to redress from the courts if they are unjustifiably discriminated against because of their age."