Officers policing the controversial badger culls in England have been accused of "highly questionable" actions by civil liberties lawyers, after they told protesters their personal details would be passed to the National Farmers' Union and handed out NFU-produced leaflets about the union's civil injunction.

The cull, aimed at curbing tuberculosis in cattle, has prompted the biggest animal rights protest since the ban on fox hunting with dogs, with accusations of intimidation from both sides. The NFU, which represents the private companies carrying out the cull, obtained a civil injunction prohibiting harassment of people linked to the cull, as well as the blowing of whistles and waving of torches to disrupt shooting.

Gwendolen Morgan, a solicitor at Bindmans, said: "The police's actions here appear highly questionable. The passing of personal information to a private body, without any apparent legal basis, is a serious concern. Since when did the police become the enforcers of the NFU's civil injunction?"

In a video filmed last Thursday in Gloucestershire passed to the Guardian, a police officer tells a protester: "Because you have obviously been in the vehicle whilst the alleged offences happened. OK, your details will be passed to the National Farmers' Union, and it is possible you may now be subject to the civil injunction they have taken out … but that will be up to the National Farmers' Union as to whether they wish to put that through court."

The protester told the Guardian: "The police say they are being independent but this looks like they are acting as a private security firm for the NFU." He was not arrested and those arrested on that night have now been released without charge.

Officers have also been giving protesters NFU-produced leaflets that state a breach of the civil injunction may lead to "summary arrest without warrant" and that "you are hereby put on notice of the injunction". In an earlier incident recorded on 6 September, an officer is asked why some protesters are being detained. He replied: "The NFU are coming down to give them an official warning because they committed aggravated trespass."

Morgan said: "There are major civil liberties issues at play. If the police are exercising their powers to satisfy a private interest group, that would be very improper and very concerning to the majority of peaceful citizens concerned about the cull. It seems like the police are going beyond their powers and doing more than is permitted."

A spokesman for Gloucestershire police said: "The officer in this case should have stressed that we can pass information onto the NFU, rather than will. As the high court issued the injunction and the NFU would not be able to enforce it without details of possible breaches, we believe in certain circumstances there is a pressing social need to disclose information." The police also said their communication had been "unclear" regarding the 6 September incident and said the NFU have no power to issue official warnings.

Regarding the distribution of NFU legal leaflets, the spokesman said: "To help those who do wish to protest we feel it is important they are aware of the injunction so that they can ensure any action they do take is legal. Therefore protestors have been issued with an explanation of the injunction."

An NFU spokesman said: "The injunction granted by the high court does not prevent peaceful legal protest. The injunction is about stopping people who are intent on harassing and intimidating farmers and landowners going about their lawful business. Families have been subject to intimidation, with alarms set off during the night, abusive phone calls through the night, threatening letters and messages, and strangers verbally abusing them at their own homes."

The NFU's relationship with the government is also causing controversy, after the department for environment, food and rural affairs (Defra) refused to disclose information on the cull on the grounds that communications with the NFU constituted "internal communications". The Information Commissioner has ruled against Defra's refusal, but the department is appealing the decision and on Thursday, Morgan, who acts for the Badger Trust, filed new legal documents challenging Defra's appeal.

"If Defra were to succeed in their use of the 'internal communications' exemption, this would be a very worrying development for democratic decision-making," said Morgan, adding that it would set a precedent of secrecy for any lobbyists' communication with government. "The Badger Trust is going above and beyond the call of duty and taking on the wider case on behalf of all those organisations who campaign for transparency in decision-making."

A Defra spokeswoman said: "Defra is appealing the commissioner's decision as the information requested includes risk registers that are still relevant to the ongoing badger cull pilots." The department will argue that because its officials discussed the risks with the NFU on behalf of the cull operators, those communications can be considered internal. Defra's appeal will be heard on 18 December.

Jeff Hayden, a director at the Badger Trust, said: "The NFU is a lobbying organisation and should be seen and treated as such."