People convicted of child sexual abuse being held at an immigration removal centre have been inadequately supervised, a watchdog has found.

In one case, a detainee was transferred to prison following an allegation of grooming, HM Inspectorate of Prisons (HMIP) said after an unannounced inspection of the privately run Colnbrook IRC in west London, near Heathrow.

“There had been inadequate supervision of some detainees who could have presented risks to children and others in the visits room and around the centre,” the inspectorate’s report said.

Inspectors examined the records of two detainees who had been convicted of sexual offences against children. Both of them had notes placed on their electronic records to inform staff that visits should be supervised. However, staff supervising visits were unaware that these records should be checked or of the risk the detainees could pose to children and others, particularly during visits to the centre.

“No other measures were taken to supervise these detainees, one of whom was subsequently found downloading inappropriate content in the internet room,” the report said. “He was banned from further use of the facility, but no further restrictions were placed on him. He was transferred to prison some days later, when an allegation was made that he was grooming another detainee.”

HMIP reported that the centre was largely indistinguishable from a prison, with male detainees locked in cells for long periods, staff reporting that the availability of drugs was a problem and a threefold rise in self-harm since 2016.

Conditions were described as “austere for most prisons”, with poor ventilation, sealed windows and limited outdoor space. The Home Office is planning to build a new centre to replace Colnbrook and the neighbouring Harmondsworth.

Detainees at Colnbrook, run by facilities management company Mitie, had been held for an average of 75 days, longer than at other centres. Seven had been in detention for more than a year, including two for more than 24 months, inspectors reported.

Some had been UK residents for many years, including an ex-offender who had lived in the UK since the age of three months whom the Home Office was seeking to deport.

After examining 12 cases, HMIP concluded that the Home Office had “failed to act diligently or expeditiously” in five of them.

A Home Office spokesperson said: “We do not detain people indefinitely, and the law does not allow it. In 2018, 92% of those detained left detention within four months and 69% in less than 29 days.

“We have always been clear that we expect the highest standards from detainee custody officers and others who work with detainees, and we are working closely with Mitie to make sure that all high risk detainees are properly supervised at all times.”

A Mitie spokeswoman said: “To ensure adequate supervision during family visits, adults with children now have a designated area in the visiting hall.

“Other detainees are not permitted in this area. Detainees known to be a potential risk due to previous offending behaviour are allocated visiting areas adjacent to staff to ensure they are adequately supervised.”

An inquest found last month that a catalogue of failings contributed to the killing of a man at Colnbrook in 2016. His killer – who was known to mental health services and had 16 previous convictions for 33 different offences – should not have been placed in detention, a jury concluded.