A year-old immigration removal centre was slammed by inspectors as "fundamentally unsafe" with "serious" problems, including bullying, violence and drugs, according to a report published today.
Brook House at Gatwick airport opened in March last year and holds around 400 men, most of whom are awaiting removal from the UK.
But inspectors said challenges posed by opening a new centre and dealing with ex-prisoners and difficult detainees were no excuse for the problems identified during the inspection in March.
Dame Anne Owers, Chief Inspector of Prisons, said: "New custodial establishments frequently experience early difficulties as staff and detainees get used to their new surroundings and each other.
"However, by the time of this first full announced inspection, a year after the centre opened, managers could be expected to have resolved teething problems.
"Instead, we were disturbed to find one of the least safe immigration detention facilities we have inspected, with deeply frustrated detainees and demoralised staff, some of whom lacked the necessary confidence to manage those in their care."
The inspectors found only a third of detainees said they felt safe on their first night.
The centre was designed on the assumption men would stay for only a day or two, but the reality was many stayed for weeks or months, according to the report.
Inspectors highlighted insufficient activity and education provision to occupy detainees, 11 of whom had been in the centre for more than 10 months.
Staff felt "embattled" and lacked the confidence to deal with bad behaviour by detainees and the use of force was high, the report found.
Living accommodation was "noisy and institutional" while drugs were a "major concern" and inspectors were unable to identify a "systematic approach" to preventing drug supply and use.
Positive aspects of the report included the fact that those at risk of self-harm were well cared for and faith provision was good.
Dame Anne said: "The challenges of opening a new immigration removal centre should not be under-estimated, particularly with inexperienced staff and challenging detainees, many of them ex-prisoners.
"The challenge at Brook House was significantly compounded by poor design which built in boredom by providing too little purposeful activity on the erroneous assumption that detainees would be staying only a few days.
"But none of this can excuse the fundamentally unsafe state of Brook House, which must be urgently addressed by G4S and UKBA."
David Wood, strategic director for criminality and detention for the UK Border Agency, said: "We are extremely disappointed with this report, but accept its broad conclusions.
"That is why we have acted so swiftly to implement the vast majority of the improvements recommended.
"Since the inspection, we have introduced an anti-bullying policy and additional support for staff, including designated mentors.
"We are also developing a comprehensive drugs strategy for the estate to supplement the intelligence-led approach we have to preventing drugs coming in and being used in the centre.
"We are continuing to build excellent working relationships with the local police which have already led to a number of drugs prosecutions.
"The vast majority of detainees in Brook House have committed very serious crimes, including drugs, sex and violent offences.
"The centre therefore faces a number of challenges on a daily basis. We are absolutely committed to meeting these challenges and the remaining improvements will be made in the next few months."