Police and prosecutors are unable to provide statistics on the use of body-worn camera footage in court despite forces spending millions of pounds on the technology, according to a report.

Big Brother Watch said it was impossible to assess claims of the equipment's value without figures covering the role it has played in criminal proceedings.

Research by the privacy campaign group found that nearly three-quarters (71%) of UK police forces have spent a total of £22.7 million on 47,922 body-worn cameras.

This was a sharp rise compared with previous analysis, which found fewer than 3,000 cameras had been acquired for £2.2 million in 2010.

Supporters of the gadgets say they improve transparency, drive down complaints against officers and help speed up justice by encouraging offenders to plead guilty when crimes are caught on camera.

But Big Brother Watch claimed that neither the police nor the Crown Prosecution Service could provide data relating to the use of footage in criminal proceedings.

Freedom of information requests were submitted asking for details of how often video captured by a body-worn camera had been used in court, and the number of successful and unsuccessful prosecutions.

Forces said the figures were not centrally recorded, not held in an easily retrievable format or not held at all, while the CPS responded that the request would require a "manual review of case records", according to the report.

Renate Samson, chief executive of Big Brother Watch said: "The police repeatedly reassure the public that body-worn cameras will enhance transparency, create better relations, and improve prosecution rates, but despite 71% of forces rolling out nearly 48,000 cameras, these benefits are yet to be conclusively proven.

"It says little for the approach to transparency that neither the police nor the Crown Prosecution Service could tell us how often footage from the cameras has been used in court proceedings.

"If the future of policing is to arm all officers with wearable surveillance, the value of the technology must be proven and not just assumed.

"It is not enough to tell the public they are essential policing tools if the benefits cannot be shown."

Big Brother Watch called for data to be collated and published to show how often body-worn camera footage is used as evidence during court proceedings and in obtaining early guilty pleas.

"With no access to the facts and figures relating to the outcome or how footage from body-worn cameras is used, it is impossible to assess the claims of their value," the paper said.

The report is published a day after Scotland Yard announced that head-mounted cameras are being issued to firearms personnel in the capital.

In total, the Metropolitan Police is issuing body-worn video to 22,000 officers in what is believed to be the largest roll-out of the technology in the world.