G4S has suspended nine members of staff from an immigration removal centre near Gatwick Airport, following a BBC Panorama undercover investigation.
The programme says it has covert footage recorded at Brook House showing officers “mocking, abusing and assaulting” people being held there.
It says it has seen “widespread self-harm and attempted suicides” in the centre, and that drug use is “rife”.
G4S said it is aware of the claims and “immediately” began an investigation.
The security firm said it had not been provided with recorded evidence, but added: “There is no place for the type of conduct described.”
The programme, to be aired on Monday, uses footage it says was recorded by a custody officer at the centre, which holds detainees facing deportation from the UK.
Panorama says it has seen “chaos, incompetence and abuse” at the centre, which it describes as a “toxic mix”.
It claims detainees who are failed asylum seekers can share rooms with foreign national criminals who have finished prison sentences.
G4S said the staff suspensions were a “precaution” but it has reported the allegations to “the relevant authorities”.
“Such behaviour is not representative of the many G4S colleagues who do a great job, often in difficult and challenging circumstances,” Jerry Petherick, managing director for G4S’ custodial and detention services in the UK, said in a statement.
“Once we have seen the evidence and concluded the investigation, I will ensure that we take the appropriate action.”
Mr Petherick later told the BBC his company “continually look at vetting and training” and the initial training course is eight weeks, with “ongoing developmental training” after.
“We continue to focus on the care and wellbeing of detainees at Brook House,” he said.
Who are the detainees?
Brook House is currently home to 508 men – mainly from Pakistan, Albania, Nigeria, Afghanistan and India.
According to the Home Office, the majority of those held are failed asylum seekers or illegal immigrants waiting to be deported from the UK on organised charter flights.
Other detainees include foreign national offenders awaiting transfers and those who are considered too challenging to manage in less secure centres.
Brook House is one of 11 detention removal centres in England, which together took in 28,908 people last year – including 71 children.
During the year, 28,661 people left detention – of which 64% were held for less than 29 days, 18% for between 29 days and 2 months, and 11% for between two and four months.
Of the 1,848 (6%) remaining, 179 had been in detention for between one and two years, and 29 for two years or longer.
In response to the Panorama investigation, a Home Office spokesman said: “We condemn any actions that put the safety or dignity of immigration removal centre detainees at risk.
“We are clear that all detainees should be treated with dignity and respect and we expect G4S to carry out a thorough investigation into these allegations and that all appropriate action be taken.”
Brook House is operated privately by G4S on behalf of the Home Office.
The firm works in a number of sectors, including technology, care and justice services, and cash transportation, as well as consulting on security.
Brook House was opened in 2009 but a year later was branded “fundamentally unsafe”.
A further report in 2012 found there were still “significant concerns”, but in 2013 inspectors saw sustained improvement.
The most recent report from HM Chief Inspector of Prisons, released in March this year, said some detainees had been held for excessive periods due to “unreasonable delays in immigration decision making”.
The report also described the residential units as “very closely resembling the conditions found in prisons”, saying problems were “exacerbated by poor ventilation and unsatisfactory sanitary facilities”.
A follow-up review into immigration detention had already been commissioned by the Home Office before the Panorama investigation, and it will include a visit to Brook House.
Former Prisons Ombudsman Stephen Shaw carried out an inquiry into detention in the UK in 2015 and his report made 64 recommendations – including that fewer people should be held in immigration centres.
The review into progress made since will take about six months and will begin on Monday.
Watch Panorama – Undercover: Britain’s Immigration Secrets – on Monday 4 September at 21:00 BST on BBC One and afterwards on BBC iPlayer.