Detainees have caused a “disturbance” at a London immigration removal centre during a power outage, the Home Office says.
No one was injured during the incident at the Harmondsworth detention centre in West London.
It is understood that a group of detainees left their rooms and went into the courtyard at the immigration centre armed with various weaponry.
There was a power cut at the premises in the early hours of Saturday.
The power was still out at the premises in west London just before 0900 GMT and the Home Office said work was underway to resolve the issue.
The government said no detainees had left the premises and those involved had since been returned to their rooms.
Police officers and the HM Prison Service are at the scene. The Metropolitan Police said officers attended the incident at 0745 and were still there.
The detention facility in West Drayton, near Heathrow Airport, holds hundreds of men, including adult male asylum seekers, foreign offenders awaiting deportation and men who are in the UK illegally.
A government report on the Harmondsworth immigration removal centre found some concerns with the site including living conditions “below an acceptable standard”, following a scrutiny visit last year.
The chief inspector of prisons reported filthy cell toilets, problems with pests and dilapidated communal showers.
Other concerns raised included high levels of vulnerability among detainees, people assessed to be at risk of harm being held for too long and detainees being locked in their cells during lunch and overnight.
The purpose-built immigration removal centre opened in 2000 and has a capacity of about 670 people. It is run by contractors Mitie Care and Custody.
The centre has had a controversial history. In October 2012, detainee Prince Fosu, 31, was found dead on the floor of his cell, and two firms running the centre faced prosecution.
In 2018, the CPS reversed its decision saying the firms should not face health and safety charges.
Conditions at the centre were described as desolate by a 2016 report, which said some detainees were being held for too long.
It comes as the government has faced widespread criticism this week for its handling of overcrowding at an immigration centre in Kent.
The Manston processing centre came under the spotlight after reports emerged that migrants, including families, were being held for four weeks, in breach of the law.
The site, meant to hold people for no more than 24 hours, was built to hold 1,600 migrants at any one time – but Home Office minister Chris Philp said there was more like 4,000 there on Monday.
Defending the government after criticism of its handling of the situation, Mr Philp said on Friday there had been a “dramatic improvement” in conditions.
Mr Philp, the area’s MP, admitted Mistakes had been made after two groups of migrants from the Manston centre had been left Stranded in London.
He told Sky News both groups had told immigration officials they had addresses to go to but “that turned out subsequently not to be the case”.
“Quite how that misunderstanding arose, maybe it was lost in translation, I don’t know, but clearly they have now all been looked after,” he said.
Prime Minister Rishi Sunak described the challenge of migrants entering the UK via the English Channel as “serious and unprecedented” in an interview with the Times on Saturday.
“There’s no easy overnight fix to that challenge,” he said.
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