Inspectors have described the "worst conditions they have ever seen in immigration detention" inside a centre near Heathrow in west London.
Chief inspector of prisons Charlie Taylor was so concerned by what he found during the visit in February that he wrote to then Home Secretary James Cleverly but received no reply.
The visit to Harmondsworth Immigration Removal Centre revealed "chaos" that was "truly shocking", including violence and drug taking.
There had also been numerous suicide attempts made by people being held there, including an attempt during the inspection of the centre that led to "serious injuries to the detainee".
Inspectors found the support for those who had said they felt suicidal was "not good enough".
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A report following the visit details how a shortage of prison places meant more former prisoners were being held in Harmondsworth alongside people with no criminal convictions.
Assaults had doubled since the last inspection in 2017 and drug-taking, which is usually rare in immigration removal centres, was now "widespread".
Inspectors could "smell cannabis" and saw detainees openly smoking in communal areas without being challenged by staff who largely kept to offices with "do not enter" taped across their doors.
Nearly two thirds of those being held in the centre, which can hold close to 700 men, said they had felt unsafe there.
Mr Taylor said: "The level of chaos that we found at Harmondsworth was truly shocking and we left deeply concerned that some of those held there were at imminent risk of harm.
"Nobody should be detained in an immigration removal centre unless they are going to be removed quickly from the country, yet around 60% of detainees were released from the centre, with only a third deported, which begs the question of why so much taxpayer money was being spent keeping them locked up in the first place."
Inspectors found the centre had doubled up cells so they could hold more men and the only exceptions to this were for those at risk of serious harm.
After two days, men who refused to share a room were taken to the separation unit by a team dressed in full personal protective equipment and kept there until they agreed to share.
Mitie Care and Custody, which runs the centre, was found to be facing a number of challenges including a staffing shortfall and a "shambolic retendering process run by the Home Office, which meant that leaders had no idea whether they would still be running the centre in the next few months".
The report found the uncertainty "made it all but impossible to recruit permanent senior staff, who might only be in post for a matter of weeks".
Mr Taylor said the new director had a "good understanding of the many challenges" and was beginning to make progress but added "she will need strong and consistent support at every level to succeed".
A Home Office spokesperson said: "We take the welfare and safety of people in our care very seriously and it is vital that detention and removals are carried out with dignity and respect.
"The number of custody officers and dedicated welfare staff at Harmondsworth IRC has been increased since the inspection."
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A statement from Mitie said: ""We acknowledge that the conditions and standard of care during this unannounced inspection in February 2024, fell below the level we would expect at this facility. However, a number of critical factors outside our control impacted conditions at the IRC at this time.
"Working in partnership with the Home Office, significant action has been taken to address the concerns raised by inspectorate. It is positive to see HMIP's report acknowledge that improvements have already been made since February."
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