However, critics have warned against placing too much trust in technology.

Dr Mark Griffiths, a consultant in oral surgery at Bristol Royal Infirmary, has invented a machine he says can be used for any operation where someone is under a general anaesthetic.

It works by using electrodes to monitor changes in brain patterns - known as alpha rhythms - during surgery.

While it is very rare for anyone to come round and feel pain while on the operating table, Dr Griffiths says this new system can spot the early warning signs of inadequate anaesthesia.

They include an increase in sweating and salivation, blood pressure and heart rate and movement of the eyes.

Dr Griffiths said: "This doesn't tell us what is going on in the brain, but at least it is an indication of when someone is waking up from their anaesthetic and becoming sedated rather than anaesthetised."

Information may be misleading

However, doctors say the machine is no substitute for an anaesthetist and that, because we know so little about the brain, the information the monitor gives may be misleading.

Dr Jose Ponte, an anaesthetist at King's College Hospital, London, said: "There are great limitations in this approach to monitoring a brain function and though it's promising, too, I am quite sceptical that it will resolve the main problem that we have nowadays, which is to detect when a patient is awake."

At the moment the equipment is available only at the Bristol Royal Infirmary and Dr Griffiths so far has not been able to find a manufacturer to produce it in large numbers.

But he is still hopeful it will become standard monitoring equipment in operating theatres across the country.