The death of a man in an immigration removal centre a fortnight ago is believed to be linked to the drug spice, in what is thought to be the first case of its kind.

The death comes a year after an official report into conditions at Brook House near Gatwick Airport warned that staff believed “it was only a matter of time before a detained person died as a result of taking spice”.

The 26-year-old man, who the Observer understands was a French national, died in his cell at Brook House on 27 October. The prisons ombudsman has launched an investigation into his death and an inquest is expected to be held.

Diplomatic sources at the French ministry of foreign affairs told the Observer they are following this case closely, in conjunction with British authorities. It is the first death in immigration detention and removal for almost a year and one of 22 such deaths in the past decade.

While no conclusive cause of death has yet been publicly confirmed, in a highly unusual move the Home Office contractor Serco, which manages Brook House, circulated two letters to detainees about the death, both of which referenced drug use.

The first, which was distributed on the day the man died, stated: “This tragic event may or may not be substance-related, so I urge you not to take any uncontrolled substances.”

The second note to detainees distributed two days later said senior officials were “increasingly concerned that there are substances within the centre that could be harmful and cause residents to suffer an extreme reaction, including losing consciousness”. Detainees were urged to “dispose of these substances and not to take any uncontrolled tablets or powders”. The letter specifically referenced spice.

Two detainees who knew the dead man and spoke to the Observer on condition of anonymity said they believed the man was in a bad state mentally and had asked to be voluntarily returned to France.

The first detainee said he thought the man may have been inhaling spice through vapes and said he had often begged other detainees to help him get the vapes.

The second detainee said: “I have significant concerns regarding the increase in contraband, including illicit drugs, within the centre. Two detainees were recently found intoxicated and unconscious and required emergency medical treatment.

“As someone with PTSD [post-traumatic stress disorder], I can attest that the conditions here exacerbate mental health struggles significantly. The lack of support means detainees are left to their own devices, with many turning to drugs as a means of coping. The rates of violence and substance dependency have reached alarming levels, contributing to the already challenging environment.”

Brook House hit the headlines in 2017 after an undercover exposé by the BBC’s Panorama revealed a catalogue of abuse of vulnerable detainees. A public inquiry was subsequently held into conditions at the centre and part of the evidence heard related to problems with spice. The inquiry report, published in September 2023, told of “spice, drugs, people collapsing, emergencies, people looking like zombies piling into each other”. It was not uncommon to see people “wetting themselves, collapsing and frothing at the mouth”.

The report states: “One member of the medical team and another staff member remarked that it was only a matter of time before a detained person died as a result of taking spice.”

Emma Ginn, director of the charity Medical Justice, which works to support the health of immigration detainees, said: “It is deeply concerning that Brook House contractors have mentioned drug use in relation to the death of the 26-year-old man. Immigration detention is particularly harmful for vulnerable people who find themselves locked up indefinitely and we call for its use to be ended.”

A Serco spokesperson said: “The safety and wellbeing of the residents in Brook House is our first priority and we are constantly working to stop the illegal entry of drugs into the centre and prevent any further tragedies. This is always challenging and we use a combination of technology, specialist dogs and intelligence-led interventions.”

A Home Office spokesperson said: “The Home Office has always taken a zero-tolerance approach to drug use and we continually review and strengthen procedures to ensure illicit substances do not enter our centres.”