No blame should attach to prison staff over the death of serial killer Harold Shipman, a senior member of the Prison Officers' Association has said.
Standard watch means frequent but irregular checks
Steve Cox said: "If we had 78,000 officers, we could probably stop every prison death."
"There is no blame because there are no resources to watch prisoners 24 hours a day."
Shipman was on a standard security watch at Wakefield Prison in West Yorkshire at the time of his death.
Harry Fletcher, Assistant General Secretary of the National Association of Probation Officers, told BBC News 24 this regime meant frequent but irregular checks at intervals of between 40 and 90 minutes.
"But we're talking here about somebody who is very skilled in medical matters, and if he was determined to commit suicide, which clearly he was, then he would," Mr Fletcher said.
Prisons Minister Paul Goggins has announced he is putting Prisons and Probation Ombudsman Stephen Shaw in charge of the investigation into Shipman's death.
Mr Shaw, who was not due to take formal responsibility for death in prison inquiries until April, also said there was a limit what staff could have done.
"You have 70,000 people in prison. You do not have a situation, nor would you seek a situation, where they are all under 24-hour surveillance. That isn't possible, it isn't desirable, it isn't humane," he told BBC Radio 4's The World
At One programme.
He said he would need to investigate whether or not there had been any warning signs or slip-ups in terms of procedures.
Most prisons operate a system of privileges - basic, standard or enhanced - which can be earned or lost depending on a prisoner's behaviour.
Over Christmas, Shipman was put on basic privileges after being unco-operative with staff.
He was moved back to standard-privilege level a week before he died.
At Wakefield Prison, the basic level means lock-up at 6.30pm Monday to Friday, £3 in private cash, wearing prison uniform and no TV in the cell.
Inmates on standard level enjoy lock-up at 8pm, £12.50 in private cash, their own clothes and a TV in their cell, paid for by themselves.