As a result many dying patients do not get appropriate palliative - end of life - care which could help them live out their lives in comfort, said the team of researchers from the Cross Cancer Institute in Edmonton, Alberta.

Dr Antonio Vigane, who helped to lead the study, said: "The major problem with clinical estimation of survival appears to be its lack of sensitivity in identifying patients with poorer survival, for whom an adequate delivery of palliative care would be more appropriate.

"It is possible that a better knowledge of patients, in combination with some other possible predictors of survival, may improve prediction."

The researchers, whose work is reported in the journal Cancer, studied the cases of 233 terminal cancer patients, and found that doctors overestimated how long they would live in 52 per cent of cases.

Over optimistic

Chemotherapy can be toxic

Jeanette Webber, chief nursing officer for the Cancer Relief Macmillan Fund, said some UK doctor were also prone to be over optimistic in their diagnosis of how long cancer patients had left to live.

She said in some cases doctors did not tell patients that they were terminally ill because they knew the patient would not be able to take in the news, and might reject palliative care.

But she said in other cases doctors refused to accept defeat, and obstinately continued to look for an effective treatment even when it was plain that none existed.

She said: "Things have improved significantly in recent years, but there are still people who could benefit from good symptom control and good psychological care and support who are not getting it.

"Some people are also being treated with chemotherapy which is quite toxic, and which is leading to a rather unpleasant period prior to death with almost no chance of doing them any good.

"But some patients, even if they know chemotherapy will probably do them no good, still opt for it, and in those cases doctors are probably doing the right thing because it is what the patient wants."

Ms Webber said it was important that medical staff got as close to patients as possible so that they could take treatment decisions based on a careful analysis of the patient's needs.

She said it was also vital that doctors learned when to accept there was nothing more they could do for patients.

The researchers underestimated in 23 per cent of cases and got it right 25 per cent of the time.

In developed countries, approximately 30 per cent of the population will develop cancer.