Immigration detainees are being held for "excessively long" periods in "prison-like" conditions, the prisons watchdog says.
It said Home Office failings were sometimes to blame for the prolonged detentions at Harmondsworth Immigration Removal Centre, near Heathrow.
In some cases, a removal to another country failed because a lack of travel documents or a late legal challenge.
The Home Office said people are held for the shortest time possible.
Harmondsworth Immigration Removal Centre holds up to 676 men, which can include asylum seekers, illegal immigrants and foreign national offenders.
It is the largest detention site in Europe and, during the October inspection, it had 552 detainees.
The report by HM Inspectorate of Prisons found 23 of those had been held for more than a year and in one case, a man had been held for four years.
In 90% of cases examined during the inspection, people had been tortured before coming to the UK but remained in detention, the watchdog added.
It also found handcuffs were routinely used on detainees when attending appointments outside of the centre, even though there was no "evidence of risk".
'Below decent standards'
Despite improvements of conditions at the centre, the watchdog said many areas were dirty, bedbugs were "endemic" and there were mice.
It also highlighted an increased availability of drugs, including the synthetic drug Spice.
Detention times and conditions within immigration centres have been under scrutiny after a number of claims came to light.
Last year, a Panorama investigation uncovered claims of staff "mocking, abusing and assaulting" people being held at a centre near Gatwick Airport.
Chief prisons inspector Peter Clarke said it was the third inspection in a row with "considerable failings" 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."
The Home Office said detention and removal of those without the lawful basis to stay in the UK was an essential part of effective immigration control.
But it admitted the report made for "difficult reading" and it would make changes.
A spokeswoman said: "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."