Washington: If you're facing surgery, this may well be your worst nightmare: waking up while under the knife without medical staff realising.

The biggest-ever study of this phenomenon is shedding light on what such an experience feels like and is causing debate about how best to prevent it.

For a one-year period starting in 2012, an anaesthetist at every hospital in Britain and Ireland recorded every case where a patient told a staff member that he or she had been awake during surgery. Prompted by these reports, the researchers investigated 300 cases, interviewing the patient and doctors involved.

One of the most striking findings, said the study's lead author, Jaideep Pandit of Oxford University Hospitals, was that pain was not generally the worst part of the experience: It was paralysis. For some operations, paralysing drugs are given to relax muscles and stop reflex movements.

"Pain was something they understood, but very few of us have experienced what it's like to be paralysed," Professor Pandit said. "They thought they had been buried alive."