Release is taking legal action against the British Transport Police (BTP) to determine if the use of sniffer dogs to detect drugs is lawful. If we are successful, the case will require the police to stop using sniffer dogs for this purpose.

The case was sparked by an incident in which Release's executive director, Sebastian Saville was searched last year by the BTP at Camden Town underground station following a positive indication by a sniffer dog. Saville had no illegal drugs in his possession.

Release argue that Saville was unlawfully searched and detained, and that these actions constituted a breach of Saville's fundamental human rights of freedom of movement and respect for private life, as well as constituting a trespass to his person. These kind of civil liberties are what distinguish our own society from the authoritarian and repressive ones that we loathe and fear. Adhering to the principle that the police are here to serve and protect the public requires our police forces to tread a fine line, and sometimes this line is crossed. The use of sniffer dogs to identify people carrying drugs as they make their way through London's transport system is not only wrong in principle, but it is also ineffective in practice.

Australian research has found that in 74% of searches following an indication by a police dog no drugs were found. No equivalent comprehensive research has been conducted in the UK; however preliminary inquiries via freedom of information requests indicate that the deployment of police dogs here produces similarl results. During Operation Shelter, conducted by the British Transport Police during Latitude festival in Ipswich in 2008, only 12% of searches conducted as a result of "tells" by police dogs located illegal drugs.

Sniffer dogs are not about catching drug dealers. The dogs lack the sophistication to distinguish between someone who has been in contact with drugs and someone who's actually carrying them, let alone to determine what kind of quantity that person is carrying, and what they intend to do with it. Mr Hot Shot Dealer does not travel the tube with his stash. These dogs are not used to protect the public. They cannot be compared to metal detectors or dogs trained to identify bombs or knives, since drugs are not used as a weapon against the public. So the argument that the ends justify the means – used to defend searching thousands of visitors entering a venue on the grounds of protecting the public from an act of violence – cannot apply in the context of personal possession of drugs.

The possession of certain substances is an offence, but the manner in which the police uphold the law must be proportional to the offence committed and the outcomes their methods achieve. In the case of sniffer dogs, neither of these principles is satisfied.

More than a third of adults in England and Wales have used illicit drugs. More than 1 million use class A drugs every year. Catching individuals carrying a small quantity of drugs has no impact on these numbers. A sniffer dog operation in Cheshire recently saw the detention at a police station of 58 people, of which only four of them were in possession of drugs. This is roughly the same as arresting half the adult population of England just to identify the million or so class A drug users.

Article 8 of the European convention on human rights protects people's right to privacy – their right to be free from unwarranted interference by the state. This is a principle worth defending and is the basis of Release's case against the BTP. The use of sniffer dogs has never been debated by parliament and there is no legislation permitting their use. The only regulation comes from guidance issued by the Association of Chief Police Officers and until someone challenges and scrutinise their use the police will continue to utilise sniffer dogs.

Section 23 of the Misuse of Drugs Act 1971 gives a police officer who has reasonable grounds for believing that a person may have illegal drugs on them, permission to search that person and, if necessary, detain them for that purpose. So the question remains, does an indication by a sniffer dog give police those reasonable grounds? Release's case against the British Transport Police will be heard at the high court later this year, and it will be up to the judges to decide.