London's Metropolitan Police is currently providing over 22,000 officers with Body Worn Video (BWV), saying it will "help officers to gather evidence and demonstrate their professionalism."
The force is one of around a dozen that have tested wearable technology, motivated by a fatal police shooting in 2011 that sparked widespread riots, as well as a major study that suggested they led to a 93 percent reduction in complaints against the police.
A series of police shootings in the United States and the recent claims of rape against a French policeman have intensified an international debate about whether cameras should be used all the time.
British police say they have helped defuse tense encounters and speed up prosecutions, but the absence of a legal obligation to use them means their scope in uncovering any police misconduct could be limited.
Privacy advocates also fear that the speed of technological advancement is outpacing ethical considerations about privacy.
"While we understand the perceived transparency benefits relating to body-worn cameras, we do have profound concerns about the potential rollout of the technology for purposes beyond law enforcement," Renate Samson, head of Big Brother Watch, told AFP.
Officials such as traffic wardens and even local council litter enforcers see the "new capabilities as the solution to a broad range of problems", she said.
"We could find ourselves being filmed all the time by officials wandering the streets."
'Speeding up justice'
Bernard Hogan-Howe, Chief Commissioner of the Metropolitan Police, began a trial of body-worn video cameras in 2014 after the death of Mark Duggan, who was shot by officers in north London in August 2011.
The death led to riots in London and other major cities, and the police chief said the use of cameras would aid investigations into police shootings.
However, the fatal shooting of Yassar Yaqub by West Yorkshire Police marksmen during a car chase last month was not caught on camera despite a force-wide rollout of the devices.
"We hope the Independent Police Complaints Commission will interrogate why body cameras were not used... particularly as the operation, by the force's own admission, was 'pre-planned'," said Just Yorkshire, a rights group.
Home Office guidelines state that "the decision to record or not to record any incident remains with the user", adding only that "failing to record an incident is likely to require explanation in court".
Hogan-Howe said the trial of the monitoring equipment in London revealed that "people are more likely to plead guilty when they know we have captured the incident on a camera... speeding up justice."
The trial also "proved particularly successful in domestic abuse cases", the police force has said.
'People do have anxieties'
The Metropolitan Police also cited a year-long study of almost 2,000 officers across British and US forces from last year, which found that the introduction of wearable cameras led to a 93 percent drop in complaints made against the police by the public.
The University of Cambridge study suggested the cameras result in behavioural changes that "cool down" potentially combustible encounters.
Deborah Coles from Inquest, which campaigns for police accountability, gave the cameras a "cautious welcome", but said they were "not a panacea".
"It's up to the government to ensure police are using them properly... and not turning them off," she told AFP.
In Britain, the cameras are attached to the officer's uniform, and those interacting with the police are informed before recording starts. They can ask for filming to be stopped, but the police need not comply with the request.
The footage from the credit-card-size camera is automatically uploaded once the device is docked, and video not retained as evidence is automatically deleted within 31 days.
If it is considered relevant, however, the footage can be stored indefinitely on servers at Microsoft data centres, raising further questions about privacy.
Suspects can obtain the footage under freedom of information law, but campaigners worry that the public has not been informed properly of their rights.
"The majority of people do have anxieties about the use of cameras," said Samson, of Big Brother Watch.
"Surveillance capabilities are only increasing, yet the conversation with the general public hasn't improved."
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