Police and the prisons ombudsman are investigating the death of a 16-year-old boy who was being held at a young offender institution in south-west London.

The boy was found unresponsive at Feltham YOI on Monday night. He was treated by paramedics before being rushed to hospital but died shortly before midnight.

Deaths of children in YOIs are rare: the most recent one in England or Wales before this week was in June 2019. A Metropolitan police spokesperson said: “At this stage, the death is being treated as unexpected. A postmortem examination is scheduled for Friday 24 April. Officers are working at pace to establish the full circumstances and are pursuing all lines of inquiry.”

Deborah Coles, the director at Inquest, a charity supporting families who have lost loved ones in the care of the state, said: “No child should be dying in the care of the state, let alone a prison.

“It is clear that imprisoning children will always be harmful and never be safe. [This] death is a urgent reminder of this. The government must divest money away from punishment and prisons and into community-based support services to prevent further death and harm.”

Enver Solomon, the chief executive of the social justice charity Nacro, said: “We are incredibly saddened to hear of the death of a child in Feltham YOI. Our thoughts are with his family at this tragic time.

“While we don’t yet know all the details of the circumstances around the death, it is clear that the youth custodial estate is not fit for purpose.

“We hear time and time again about safety concerns and high levels of violence. The government must expedite its reforms of the youth justice system that must include a plan of how it will close prisons like Feltham and introduce smaller, secure children’s homes which are more suitable, effective and safer.”