An inquiry has begun to establish how two female prison officers became ill amid fears their tea may have been poisoned in jail.
The victims became seriously ill and collapsed at Bullingdon Prison in Bicester, Oxford, on Monday, the Prison Officers' Association (POA) said.
But both women are expected to make full recoveries and have been discharged from hospital pending tests to establish the cause of their illness.
The all-male Bullingdon Prison has more than 1,000 inmates and the officers were working on the "enhanced unit" of the jail, where the inmates have extra privileges, when they were taken ill, the POA union said.
Glyn Travis, of the POA, said: "Two young female officers who were drinking their own tea became extremely ill.
"They were taken to hospital by ambulance.
"Police were called because it was suspected that their two drinks were poisoned.
"Both became extremely ill and we have been very fortunate that neither of them have had a severe reaction. There could easily have been a fatality."
A Ministry of Justice spokesman said: "Two prison officers from HMP Bullingdon were taken to hospital on September 13 2010 after becoming ill. Both have now been discharged from hospital pending tests to discover the cause of their illness."