Some 25 prison staff have fallen ill - including officers rushed to hospital - after a curry was allegedly spiked by inmates making the dish with the 'zombie drug' Spice.
The Criminal Justice Workers Union described it as a 'mass poisoning of staff' at the high-security HMP Swaleside on the Isle of Sheppey in Kent.
It is suspected the synthetic drug Spice - which can leave users in a zombie-like state - was added to the canteen meals.
Paramedics were called at 2.10pm last Thursday and set up a medical incident unit at the jail, while at least three officers were taken to hospital for further treatment.
There have now been demands for HM Prison and Probation Service to launch a 'full investigation' into the incident.
The Criminal Justice Workers Union described it as a 'mass poisoning of staff' at the high-security HMP Swaleside (pictured) on the Isle of Sheppey in Kent
CJWU official Andy Hamlin said prisoners working in the canteen at HMP Swaleside in Kent spiked a 'staff special curry'
Andy Hamlin, an official for the Criminal Justice Workers Union, said: 'Prisoners preparing food in the staff canteen spiked a "staff special curry" with what is believed to be a New Psychoactive Substance.
'Twenty-five staff were affected and were witnessed under the influence of NPS.
'Among those affected there were symptoms including breathing difficulties and raised heart rates.'
South East Coast Ambulance Service (SECAmb) sent nine ambulances to attend the incident with a makeshift incident unit set up within the prison.
A SECAmb spokesman said three people were taken to Medway Maritime for further checks, although the CJWU believed five had needed hospital treatment.
A spokesperson for the CJWU executive committee said: 'Although it is not unusual for prisoners to be working staff canteens, they are expected to be a low risk, which clearly hasn't happened in this case.
'The CJWUnion demand HMPPS carry out a full investigation into this incident.
'HMPPS and local senior management team at HMP Swaleside must do more to protect the health and safety of its staff.'
The CJWU says prisoners at the jail are employed to prepare and serve the food in the staff canteen and to hold this position they are required to complete an employment risk assessment.
HMP Swaleside, which holds men convicted of serious offences and those who pose a high risk to the public, has about 1,100 inmates.
Its last inspection by His Majesty's Inspectorate of Prisons found levels of violence were high and drugs were easy to get hold of.
More than 40 per cent of prisoners are serving sentences of more than ten years, with a similar proportion serving indeterminate sentences.
One unit is specifically allocated for men convicted of sexual offences.
The Ministry of Justice said: 'A number of staff were taken ill following an incident at HMP Swaleside last week and those who attended the hospital were soon discharged.
'There is an ongoing investigation and it would be inappropriate to comment further at this stage.'
The department was liaising with all relevant agencies involved while no one taken to hospital was in critical condition, the MoJ added.
A prison riot - the fifth insurrection in UK jails in two months - broke out at Swaleside in December 2016, with inmates smashing up a wing and starting fires.
Pictures emerged of hooded prisoners brandishing makeshift weapons, before specialist ‘Tornado’ squads of former SAS soldiers were sent in to quell the rioters and reclaim A Wing which had been seized.