French researchers selected 9000 people over 65 and found that within a decade 632 developed dementia. They found that general anaesthetics had been given to 22.3 per cent of those with dementia but 18.7 per cent of those without.

The team said at the weekend that, after adjusting for other factors, it was found that the older people who had been given at least one general anaesthetic were 35 per cent more likely to develop dementia.

Studies in mice have found that anaesthetics cause changes in key proteins known to be involved in the development of Alzheimer's disease.

Francois Sztark of the University of Bordeaux, who led the study, said: "General anaesthesia often cannot be avoided regardless of patient age. However we need other studies to understand the mechanisms involved in anaesthesia-induced neurotoxicity, and to develop strategies for avoiding (it)." Peter Venn, of the Royal College of Anaesthetists, said the link was still "a little vague" but elderly patients should be aware of the risks.

"People reach crisis point in life and often they are admitted to hospital. They've been living on a frailty knife edge, they fall and break their hip, that crisis point comes and things go downhill. General anaesthesia and surgery get caught up in that and it's quite difficult to unravel cast-iron causative factors."

He said patients should be assessed before operations and receive better aftercare.

"(Patients) need careful post-operative monitoring; putting them in a side room of a ward with someone looking in every couple of hours is not as good as putting them in a high-dependency ward," he said.

The Alzheimer's Society, a British charity funding research into the links between surgery and dementia, agreed it was important for elderly patients to be watched carefully after an anaesthetic.