Waking up as the surgeon's knife plunges into your flesh - fully aware, in unimaginable agony, yet completely paralysed - is almost too horrific to contemplate. It probably sounds like some urban myth, like something that never really happens to real people in real life, but the awful fact is that it does happen in real life. Not that often, but often enough.

Laura Cook, a 12-year-old girl from Kent, is the latest person to come forward with such a story. She was fully conscious for up to 40 minutes, although paralysed by muscle-relaxing drugs, during an operation to remove her appendix. She could hear the surgeon asking for the implements she knew he was going to cut her with.

As chance would have it, something very similar happened to my mother when I was born by caesarian section. She came round to feel something that felt very much like her innards being ripped out. She tried to scream, but couldn't raise her voice above a muffled whimper. Eventually, she thinks, the anaesthetist noticed the tiny movements she managed to make with her eyes and sent her back under.

My mother did not report her experience, but Laura's parents did - and are now planning to sue.

As with many of these disturbing cases, it appears that the anaesthetist concerned, Dr Joseph Balzan, failed to ensure a continuous supply of anaesthetic gas during the operation (the other reason commonly given for these incidents is that the supply of anaesthetic ran out without anyone noticing). So the paralysing agent that Laura was given, intravenously, worked, but she didn't get enough of the anaesthetic gas through the tube down her windpipe.

Laura's parents are disgusted that Dr Balzan is still working at the Princess Alexandra Hospital, where it is not normal policy to suspend a doctor in cases of human error. The General Medical Council has said it will be at least six months before any potential misconduct charges could be brought.

Professor Peter Hutton, president of the Royal College of Anaesthetists, is keen to point out that it is "extremely uncommon" to be "aware" during an operation, although he does say that he cannot give an exact number because "the majority of cases are settled without anyone knowing anything about it".

Hutton estimates that the incidence is around 10 per million operations. This is a small figure, but it still means that if every person in Britain were to have an operation at some stage in their life, it would be, for 600 of us, a truly agonising ordeal.

Hutton says the biggest risk is in having a caesarian, as my mother did, because the dose of anaesthetic must be kept as low as possible.

"The difficulty is that you're trying to anaesthetise the mother but not the baby, but they're joined by a common blood supply," he says.

He also points out that administering an anaesthetic is a precise art. After an initial injection to put the patient to sleep, anaesthetic levels are carefully monitored during the operation to suit the individual patient. This is done by monitoring four key indicators: blood pressure, pulse, sweating and tearing (eye watering).

If a patient is not getting enough anaesthetic, their pulse and blood pressure will start to rise, and, ideally, someone should spot that and increase the amount of anaesthetic. If these signs are missed, the doctors may not realise something's wrong until the paralysing agent starts to wear off after half an hour or so.

Stephanie Ellis, of Patient Concern, says: "We get up to a dozen complaints a year, although it is less likely to occur now than ever before. The problem is that there is no such thing as an entirely safe anaesthetic. It is a balancing act."

Another problem, from the patient's point of view, is that the ordeal may be dismissed as fantasy. "Some anaesthetists believe these experiences are just vivid dreams stimulated by voices the patients can hear around the table," says Ellis. "When patients get to us it is usually because they are angry about how the complaint was dealt with by the health authority rather than the actual effects of the drug and operation."

For those who do go through such an ordeal, the impact may be severe. Laura has been diagnosed with post traumatic stress disorder since her operation. "Quite often people who have gone through this experience require counselling afterwards as they develop a phobia about going to sleep," says Hutton.