Patients could suffer from careless whispers

And although their later recollection of the comments will only be subconscious, it could have detrimental effect on them.

"The danger is that if people overhear something, for instance that their condition has a bad prognosis, that might affect their recovery or at least their mental health," said Jackie Andrade of Sheffield University.

She was speaking at the British Association's Festival of Science in Sheffield, UK.

The new research shows that even under general anaesthetic, people undergoing surgery can hear and remember what is happening around them.

Role of adrenaline

Dr Andrade cautioned that more research needed to be done to confirm her findings but added that reassuring messages during an operation - perhaps played on a tape - might actually help a patient.

Her team of researchers played a list of words to volunteers given anaesthetic to see if they picked up unconscious memories. When the volunteers came round, they were tested to see if the right prompting made them think of the words they had heard.

Surprisingly, only those who had been actually operated on, rather than just sedated and left alone, formed the unconscious memories.

Dr Andrade said: "What we think is happening is that surgery involves tissue damage which increases the release of adrenaline, and it's the adrenaline that enables people to learn even when they are unconscious."

Experiments on rats had previously shown a link between the hormone and memory function.