Hundreds of police vans are to be fitted with closed-circuit television cameras to address concerns about the "hidden" abuse of suspects.
Scotland Yard Commissioner Bernard Hogan-Howe promised the £4m refit of all Metropolitan Police vans during a meeting with the family of Sean Rigg, a mentally ill man who died in controversial circumstances in police custody in August 2008.
The Commissioner is also expected to announce an independent commission into the way that police respond to cases involving mentally ill people. It follows concerns that officers were left to deal with some of society's most vulnerable people without proper back-up from health agencies.
Mr Hogan-Howe has said he believes cameras in vans will both prevent police wrongdoing and help identify false claims against officers.
His commitment follows allegations that a police officer racially abused a black man arrested in the aftermath of the London riots. The abuse was allegedly captured by mobile phone as there was no recording equipment fitted inside the van. The officer, Constable Alex MacFarlane, 53, who denies racially abusing the suspect, faces trial in October.
The issue of cameras was further highlighted during the inquest of Mr Rigg last month, when a jury rejected the evidence of officers about what happened in the back of the van where he was held. Mr Rigg, 40, died in Brixton police station less than an hour after being restrained and arrested by four officers in south London. His death was in part caused by an "unnecessarily" long restraint with "unnecessary force", according to the inquest's jury this month.
The original Independent Police Complaints Commissioner (IPCC) concluded in 2010 that the "officers adhered to policy and good practice by monitoring Mr Rigg in the back of the van following his arrest".
This was based on the testimonies of the officers themselves, who said that Mr Rigg was fine, but was violently "spinning himself around" the tiny caged area at the back of the van.
But the inquest jury rejected much of the police evidence and found that Mr Rigg was in a V position in the well of the van for around 15 minutes, during which time his physical and mental health deteriorated. They concluded that he was "extremely unwell and not fully conscious" by the time he was taken out of the van.
Last night, Mr Rigg's elder sister, Marcia Rigg, welcomed the CCTV announcement, but said: "These cameras must be working and they must be monitored by people independent of the police. The evidence heard at the inquest proved that what the police officers said about my brother was not possible; a camera in the back of the van would have dealt with these issues straight away."
The Metropolitan Police has just under 1,000 vans as part of its 6,000 vehicle fleet.
Mr Hogan-Howe said: "We expect to start before Christmas. It takes a while because there are a lot of police vans but I would expect by the spring for it to be complete." He added that the force would not make decisions on disciplinary action over the Rigg case until the IPCC had completed a review of its original bungled inquiry and a new investigation was completed into a custody sergeant who was found to have lied under oath at the inquest.
Estelle du Boulay, of the independent community group Newham Monitoring Project, which has campaigned to have cameras put in the back of police vans, said Mr Hogan-Howe needed to give assurances that individual officers could not control the filming and that any footage could be preserved for a sufficient period of time to ensure any potential victims could get access to it.