Foreign nationals have been kept in Britain's largest immigration removal centre for as long as four years, a watchdog report reveals.
Detainees were held in "prison-like" conditions at the facility near Heathrow Airport for unacceptably lengthy periods, inspectors said.
They found planned removals failed to materialise due to late legal challenges or a lack of travel documents, while some detentions were prolonged by Home Office failings.
The findings were detailed by HM Inspectorate of Prisons in an assessment of Harmondsworth immigration removal centre (IRC) in west London.
It is the largest detention site of its kind in Europe, holding up to 676 male detainees.
At the time of the inspection in October, the centre had a population of 552.
HMIP's report said: "Some detainees had been held in detention for too long, with 23 held for more than a year at the start of the inspection; one had been held for more than four and half years.
"Removals failed for a variety of reasons, including late legal challenges, a lack of travel documents and disruptive behaviour on removal.
"Some detentions were prolonged by failings of the Home Office and its contractors. For example, the Home Office took over a year to decide an asylum claim."
IRCs are used to house foreign nationals including overseas offenders and asylum seekers either while their cases are decided or prior to their removal from the UK.
Instances where individuals have been held for several months or years have prompted calls for a time limit on detention.
Publishing the report on Harmondsworth, chief inspector of prisons Peter Clarke said: "The continuing lack of a time limit on detention meant that some men had been held for excessively long periods."
In other findings, the inspectorate said:
- Physical conditions at the centre had improved but many areas were dirty, bedbugs were "endemic" and there were infestations of mice in some places.
- There was evidence of an increase in the availability of drugs, including the psychoactive substance known as Spice.
- In nine out of 10 cases examined, the Home Office accepted evidence that detainees had been tortured prior to coming to Britain but maintained detention regardless.
- Detainees were routinely handcuffed when attending outside appointments without "evidence of risk".
Mr Clarke concluded that the centre had failed to progress sufficiently.
He said: "For the third consecutive inspection, we found considerable failings in the areas of safety and respect.
"Detainees, many identified as vulnerable, were not being adequately safeguarded. Some were held for unacceptably long periods.
"Mental health needs were often not met. Detainees were subject to some disproportionate security restrictions and living conditions were below decent standards."
A Home Office spokeswoman said: "Detention and removal of those with no lawful basis to stay in the UK are essential parts of effective immigration controls.
"When people are detained, it is for the minimum time possible and detention is reviewed on a regular basis.
"The detainee's welfare remains of the utmost importance throughout.
"We are pleased that the inspector noted a number of positive areas of work such as welfare services, faith provision, generally adequate health care provision and positive engagement with third sector groups who support detainees.
"However, elements of this report make for difficult reading and we are committed to a programme of transformation."
Harmondsworth IRC is run for the Home Office by Mitie's Care and Custody division.
A spokeswoman for the firm said: "The inspectors' report confirms progress is being made, although there are still some things we need to do better.
"We are encouraged that, among the key inspection outcomes, the report identified low levels of violence and self-harm and that restraint was used proportionately.
"However, we are concerned that, despite the low risk to detainees, many said they did not feel safe.
"We are working to understand the extent to which their anxiety relates to their potential removal from the country as opposed to their perceptions of the centre and how we can best provide reassurance about their time in our care."
In relation to bedbugs, it is understood furniture in affected areas has been replaced. Units were deep cleaned and refurbished after the inspection.
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