A detainee has been found dead at Colnbrook immigration detention centre, Politics.co.uk can confirm.
While neither the centre nor the Home Office were able to provide information, Politics.co.uk has seen an email sent out to immigration centre staff saying that the death occurred today.
The detainee was found "unresponsive in his room". Politics.co.uk has the name of the detainee but is not reporting it yet until it knows the family has been notified.
Troublingly, the death took place while the detainee was under "constant watch". This means the detainee should be under permanent supervision because they are considered at such a high risk of attempting suicide.
A detention centre guard, speaking on condition of anonymity, said:
"For it to happen on a constant watch is absolutely unheard of, completely unheard of."
Deaths at immigration detention centres often spark protests and riots at that centre and others.
When Rubel Ahmed died in Morton Hall detention centre in September 2014, detainees started a protest, forcing staff to retreat behind a secure fence and call in a Tornado unit, which is used to quell institutional disorder.
Guards will have been told to be particularly vigilant in case detainees react angrily to the news.
Detainees at the centre are understood to have been told of the death but not of the identity of the detainee.
The death comes a month after the long-awaited Shaw review into detention centres concluded that numbers should be reduced "for reasons of welfare".
It found that the process of indefinite detention with heavily-restricted access to a lawyer was mentally traumatic for many detainees and that there should be a "presumption against detention" for victims of rape and sexual violence, people with learning difficulties, and those with post-traumatic stress disorder.
He also found that the academic literature "demonstrates incontrovertibly that detention in and of itself undermines welfare and contributes to vulnerability".
The report led to promises of significant reform from Home Office ministers, but campaigners say today's death shows it is not happening fast enough.
"Even though we don't know all the circumstances, this is another death in the detention estate and it underlines the urgency of this issue," said Paul Dillane, executive director of UKLGIG, a human rights charity which focuses on LGBT people in detention.
"The government has had the Shaw review for several months. They promised transformative reform and yet we see another death today. We need to see the consequences of this reform – and hopefully it's genuine reform – very, very quickly."
Zoe Gardner of legal support group Asylum Aid added: "If this detainee has died while supposedly under constant surveillance because of being recognised as a suicide risk, this is yet another utterly damning piece of evidence against the ability of detention centre staff to appropriately care for such people in such desperate situations and states of mind.
"Our asylum system is riddled with failings, with applicants facing a culture of disbelief from their very first interview, often meaning that people who are entirely unsuitable for detention are locked up anyway.
"We hope the Home Office will act quickly in the wake of yet another tragic death to drastically reduce the use of immigration detention, as the most recent review as well as a recent cross-party inquiry have recommended."
A Home Office spokesperson said: "We can confirm that a detainee at Colnbrook immigration removal centre died in the early hours of this morning.
"It would be inappropriate to comment further whilst the police, coroner and Prisons and Probation Ombudsman conduct their enquiries."