COVERT surveillance by animal rights groups used to prosecute alleged illegal hunting activity could be thrown out of court for breaching the European Convention on Human Rights, it has emerged.

New guidance was issued by the Crown Prosecution Service (CPS) after suggestions that the Regulation of Investigatory Powers Act, which limits the spying powers of public authorities, should apply to the activities of third parties such as the League Against Cruel Sports.

While the CPS said no authorisation under the Act was necessary where groups conducted "surveillance for its own purposes", permission would be required where police intended to use the material in court.

Tim Bonner, spokesman for the Countryside Alliance, said the Regulation of Investigatory Powers Act (RIPA) was introduced "to protect people from unjustified intrusion into their lives".

He suggested the new advice from the CPS called into question the validity of third party surveillance, which was not authorised, and believed it would be unreliable in bringing prosecutions as a result.

"The rural community does not object to proportional and justifiable investigation of criminal activity, but we believe the covert surveillance carried out by animal rights groups is neither of these," he said. "RIPA and the Human Rights Act provide protection if the state involves itself in such activity."

Police forces across the country have been largely reliant on video evidence from animal rights groups to bring the few prosecutions to reach the courts since the introduction of the Hunting Act.

A case against a member of the South Dorset Hunt collapsed last week. It was said that hunt monitors had set up miniature surveillance cameras covering badger setts without permission of landowners.

Council use of RIPA powers has been controversial in itself after officers were allowed to use spying powers to investigate noise nuisance and dog fouling as well as more serious offences of benefit fraud.

The CPS guidance states that "where the police are aware of the intention of the non-governmental organisation (NGO) to conduct covert surveillance and intend making use of the surveillance product in the event that it reveals evidence of a crime, it would be appropriate to seek an authorisation.

"This would undoubtedly be the case where the NGO is tasked to conduct the surveillance, whether explicitly or by necessary implication.

"However, no authorisation would be required where the police neither initiate nor encourage the surveillance even though they may be aware of it see." Although "the absence of an authorisation does not mean that the surveillance evidence will be automatically excluded", the CPS adds: "The fact that the evidence was obtained in breach of a Convention right is a factor which the court will consider when exercising its discretion..."

The League Against Cruel Sports responded by accusing the Countryside Alliance of "grossly misleading" the hunting community and its interpretation of the CPS guidance.

The league's chief executive Douglas Batchelor said: "We wonder how they'll respond when one of their members, facing prosecution on the basis of evidence collected by one of our hunt monitors, complains that statements from the alliance led them to believe that the monitor's evidence was inadmissible."