Macmillan Cancer Support said the number still alive in the UK at least five years after initial diagnosis has climbed to 1.25m.
But the charity says many patients and doctors do not appreciate the lasting impact of cancer and its treatment.
This, it says, is leading to long-term neglect of some patients.
Macmillan says all cancer patients should be advised about possible long term side-effects and other health problems that may develop years after treatment.
Advances in cancer treatment and improved screening mean that more people are living longer after being told they have the disease.
But even when treatment is successful, cancer patients are more prone to health problems.
These may include weight gain, memory loss, bowel problems, and bone and heart disease – often as a result of the cancer treatment.
There may also be mental health problems such as depression and anxiety.
But Macmillan Cancer Support says many people – including some doctors – are not aware of the possible long-term effects of cancer and its treatment, so patients do not get the support they need.
It estimates that around 250,000 patients who are alive at least five years after diagnosis have chronic health problems.
Professor Jane Maher, Macmillan’s chief medical officer, said failure to advise people about possible problems ahead can ruin lives.
She said: “Not managing these consequences of treatment can produce unnecessary anxiety at one end of the spectrum, and it can also cause real problems to people in terms of disabling their lives at the other end.”
She says patients need practical advice when cancer treatment ends.
“They need to know what is their risk of cancer coming back, and what is their risk of the treatment causing them problems in the future, so that they can do the things that they can do to help themselves.
“They should know what to report and who to report it to, and they can be assured that they can get back into the system to have those problems dealt with months, years, even decades after the treatment.”
In an ICM poll of just over 1,000 people, 94% said they would expect those finishing cancer treatment to be offered a full assessment of ongoing needs, and 92% said there should be a discussion about potential side effects.
A similar figure (89%) said the NHS should provide a personalised care plan.
However, Macmillan said many patients still are not offered these services.
Severe side effects
Karen Theobald, 46 and from Surrey, was diagnosed with a form of bone cancer seven years ago.
She had chemotherapy and radiotherapy, and although she has been told she will never be in remission, her cancer is stable.
However, she has suffered severe side-effects affecting her gall bladder, her kidneys and her memory.
Two years ago she developed sore lumps in her legs. She has to take morphine every day to try to control the pain.
She is grateful to be alive – but says she was not warned of the problems she encountered.
“I was told my hair would fall out, that I’d get a sore mouth and that it would affect my fertility. But that was basically it.”
She finds the loss of memory especially frustrating.
“It used to be really good, but now it’s a nightmare,” she said.
“I have to have Post-it notes all over the house. It’s the only way to get things done.”
Professor Mike Richards, the government’s cancer tsar, accepted more needed to be done to help cancer survivors with their health problems.
He said new plans, including the need to tailor care to the individual patient, were being drawn up.
He said it was particularly important that patients were aware that they could re-access services if they needed them.