A doctor has spoken of the moment he suspected a member of his medical team had administered a drug or drugs which it is alleged caused 18 patients to stop breathing.

Dr Graham Walker was giving evidence at Oxford Crown Court yesterday in the trial of nurse Benjamin Geen, of Banbury. Geen, 25, above, denies murdering two patients and causing grievous bodily harm to a further 16 while working in the accident and emergency ward at Banbury's Horton Hospital between December 2003 and February 2004.

Dr Walker said concerns had been raised after the resuscitation attempts of two men who suddenly stopped breathing after being admitted to the ward. He said Geen was on the scene on both occasions.

But he told the jury that it was not until 41-year-old Timothy Stubbs suddenly and inexplicably stopped breathing in the hospital that he formally raised concerns.

Earlier, the court heard how Mr Stubbs drove himself to hospital on February 5 suffering from stomach pains after being referred by his GP.

He was struggling to breath and stopped breathing by 7.30pm.

After he had become stable, anaesthetist Dr Kirsten May told fellow doctors she was unable to reach any explanation as to why Mr Stubbs's condition had deteriorated so dramatically.

The following day, Dr Walker raised his concerns with the risk management team and he and a team of doctors had an emergency meeting to see if there was any other possible explanation as to why so many patients had suddenly stopped breathing.

He said: "I went into that meeting hoping that I would come out of it looking like a fool. It was very distressing to come out of it having reached the conclusion that foul play had gone on."

The court also heard how traces of a muscle relaxant Vecuronium was found in Mr Stubbs's blood sample, which had not been prescribed by a doctor, as well as the sedative Midazolam.

The trial continues.