Police and managers at Britain's largest hospital have begun an investigation after it emerged that one patient had died and four others had fallen ill during surgery in the same operating theatre.

All five patients were operated on in the neurosurgical theatre at the Queen's Medical Centre (QMC) in Nottingham during the past three weeks.

The woman, aged 36, whose death has prompted the investigation had undergone spinal surgery on Thursday when her condition suddenly deteriorated in what the hospital described as "a very marked and unexpected way". She died on Friday. Sources said it was thought, at first, that the woman died as a result of an allergic reaction to anaesthetic but her death was later deemed "unexplained".

Detectives are expected to examine the possibility that either the surgical equipment or the drugs used in the theatre may have been faulty. They will also consider the possibility that equipment might have been tampered with.

A spokesman for the QMC, Europe's largest teaching hospital, would not confirm or deny last night whether anyone had been suspended in connection with any of the incidents. But staff working in the theatre at the time of the operations are likely to be interviewed.

The four patients who fell ill are now said to be well.

Peter Homa, QMC chief executive, said: "This was so unusual that we immediately looked to see if any other patient had experienced similar problems. This identified that possibly four other patients had experienced similar unexpected - though much less severe - clinical problems during surgery.

"Our own urgent inquiries continue, and we are being valuably supported by the police in the investigation of this unusual sequence of events."

All other neurosurgical patients have now had their cases delayed. All emergency neurosurgery was halted yesterday and it will start again today in a different theatre.

The hospital moved quickly to reassure all patients and their families. In a statement, it said: "Investigations will continue until we have gained a complete understanding of what occurred, and can put in place appropriate permanent safety checks or changes in practice.

"We would like to reassure all patients and their families that we are confident that the exceptional safety checks and additional measures we have introduced will ensure the safety and well-being of our patients."