Prison officers were left needing hospital treatment after a special staff curry was “spiked” by inmates.

As many as 25 employees were affected by what the Criminal Justice Workers Union (CJWU) described as a “mass poisoning of staff” at the high-security jail.

It is suspected a synthetic drug known as spice was added to the meal in the canteen at HMP Swaleside on Sheppey.

Paramedics were called at 2.10pm last Thursday (May 9) and set up a medical incident unit at the jail. Of the many officers taken ill, at least three were taken to hospital for further treatment.

Andy Hamlin, an official for the CJWU, said: “Prisoners preparing food in the staff canteen spiked a ‘staff special curry’ with what is believed to be a New Psychoactive Substance (NPS).”

NPS, which is also known as spice, are drugs designed to replicate the effects of illegal highs.

The psychoactive drug has similar effects to cannabis and can cause mood swings, anxiety, paranoia, tremors, seizures and fits.

Mr Hamlin added: “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 establishment.”

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 around 1,100 inmates.

Its last inspection by His Majesty’s Inspectorate of Prisons found that levels of violence were high at the institution and drugs too easy to obtain.

More than 40% of prisoners are serving lengthy sentences of more than 10 years, with a similar proportion serving indeterminate sentences, mostly life.

One unit is for men convicted of a sexual offence.

The Ministry of Justice (MOJ) 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.”

They added that the government department is liaising with all relevant agencies involved and that no one taken to the hospital was in critical condition.