The Prison Service has already agreed to pay damages to the prisoners in an out-of-court settlement.

Martin Narey: "Totally unacceptable"

But Director General Martin Narey said the incident was "totally unacceptable". He hoped the police investigation would lead to criminal charges against the officers involved.

The investigation follows the charging of 25 prison officers accused of brutality against inmates at Wormwood Scrubs prison in west London.

Mr Narey's move underlines his determination to clamp down on officers who mistreat inmates.

It was revealed earlier this month that officers at nine jails have been suspended amid allegations of attacks on inmates, including one who was aged under 18.

The latest incident is alleged to have happened after a prison van transferring two inmates from Swaleside Prison in Kent to High Down jail in Sutton, Surrey broke down.

The inmates claimed they were subjected to an assault after officers in another van arrived to collect them.