The government is calling on nearly 370 slaughterhouses in Britain to install surveillance cameras to help enforce legislation against cruelty to animals, following a controversial campaign run by animal rights activists.

The Food Standards Agency plans to override objections in the meat industry to the move by saying breaches of the law are "unacceptable" and enlisting the support of consumers and supermarkets for the campaign.

The move, to be discussed by the agency's board on Tuesday, represents a victory for the animal rights charity Animal Aid, which installed secret cameras in seven abattoirs across England. The charity said it had footage of pigs being kicked, stamped on and inappropriately stunned. Sainsbury's said it had suspended its contract with an abattoir in June as soon as allegations were made. It had resumed beef supplies last month once it was satisfied improvements had been made: "We have the highest standards of animal welfare and we are working with all suppliers across all species with a view to them all installing CCTV."

The abattoir owners said "comprehensive" measures had been taken, including CCTV installation and retraining of all staff with responsibility for handling animals.

The charity claimed undercover footage revealed "serious systemic problems" in a total of six abattoirs, including substandard treatment of pigs, sheep and cows before slaughter and improper stunning. The campaign group gave only one abattoir a relatively clean bill of health.

Tim Smith, chief executive of the FSA, told the Guardian he found images on the film "sickening". While he said he would not condone how the footage was obtained, he described Animal Aid's campaign as "a good thing" that had triggered a major rethink over accountability in the meat industry. "From our perspective, what we were seeing was evidence that allowed us to go directly back to the food business operators and say: 'It doesn't really matter how this footage was obtained, or how it came into our presence … look at the footage, and here is what the FSA as the enforcer in this area thinks,'" he said.

Five of the slaughterhouses filmed by Animal Aid were later investigated by the Department for Environment, Food and Rural Affairs. It dropped an attempt to prosecute a company in Devon and some of its employees after concluding there was no prospect of a conviction.

The department believed there were legal problems over the admissability of evidence obtained undercover by a freelance investigator employed by Animal Aid. He allegedly trespassed on the property to conceal a camera and obtain footage. Animal Aid, which will release a report into all seven slaughterhouses shortly, is refusing to identify the investigator. However, the Guardian has established his identity – he is a 49-year-old former hunt saboteur with a long criminal history.

Defra has now dropped all attempts to prosecute the abattoirs. It said in a statement it had dropped the prosecutions after becoming aware of similar legal precedents where courts had refused to accept "unlawfully obtained video footage".

Nearly 280 slaughterhouses in England, Wales and Scotland slaughter pigs, sheep, goats or cows and another 88 poultry.

Smith said of the undercover film: "You wouldn't want that sort of evidence being presented as any way normal, or any way being condoned by any regulator." He added: "The very disappointing bit – and I take Animal Aid at their word – is the number of [slaughterhouses] they entered resulted in the hit rate that they got … it was an unexpected result."

He added that while his agency could not compel abattoirs to install CCTV, he was backing a move to actively encourage them to do so: "We looked at the cost [of installing CCTV], and it didn't seem to us to be disproportionate, compared to the reassurance that the customer of that plant might get," he said. "If I was a major retailer in this country and thinking what I was going to give my customers that … the best animal welfare standards were being used, then I would be putting [CCTV installation] in the specification for meat procurement."

Jamie Foster, a solicitor at Clarke Wilmott LLP, who represents four of the abattoirs targeted by Animal Aid, said that where footage suggested failings, remedial action was taken. "Each of the abattoirs took that footage extremely seriously and they had to work through it with the FSA, the regulator," he said. "All the allegations have been fully dealt with by the authorities to ensure there isn't any concern."

Foster, whose legal submissions were instrumental in convincing Defra to drop its prosecutions, questioned the methodology employed by Animal Aid, which he said was breaking the law and failing to abide by statutory requirements governing how government agencies collect surveillance material.

He said that Animal Aid's investigator was recently "caught and escorted out" of another abattoir that he represents in the south-west of England. The investigator, who uses the pseudonym "Joe", was unavailable for comment.