Abattoirs and food businesses involved in meat supply are being warned to be alert to 'covert invasion' by anti-meat campaigners after an attempt to prosecute three slaughtermen was dropped by Defra.

The three men were initially suspended by their employer, Devon-based Tom Lang, in December 2009 following an undercover investigation by animal rights group Animal Aid that appeared to show improper slaughtering.

But Defra this week dropped all legal action against the men because the cases were based on "unlawfully obtained" ­video footage Animal Aid campaigners entered Lang's premises without permission and there was no realistic prospect of ­conviction.

Despite this, Animal Aid has vowed to continue its practice of undercover filming to expose cruel slaughtering. "The official monitoring system, we have shown over and over, is not working hence the need for our covert filming," said head of campaigns Kate Fowler, who denied the group had broken the law. "Our evidence-gathering exercise was entirely legitimate. It serves both the public and animal welfare."

Animal Aid's defiant stance suggested further "covert invasions" were likely, said Lang's solicitor Jamie Foster from rural ­legal experts Clarke Willmott. "What happened to Lang could happen to any business involved in meat supply."

Animal Aid hid video equipment on Lang's premises for two weeks. It claimed the films showed "animals being kicked, slapped and thrown; improperly stunned; and even decapitated while they may still have been alive", all of which are breaches of the Welfare of Animals (Slaughtering or Killing) Regulations 1995.

Foster argued the allegations were "of a technical nature" and, if monitored closely, the footage did not show actual animal cruelty. He also argued the methods Animal Aid used to gain its evidence amounted to trespass. All three slaughtermen have had their licences reinstated.