Officers have been slapped down for filming a woman at a trade union demonstration, in a court ruling which will have ramifications on the police response to protests.

The high court found police officers acted unlawfully when they filmed, photographed and took the details of a legal observer during a 'kettle' at a trade union demonstration in 2011.

The court ordered that the Metropolitan police delete the record it kept of Susannah Mengesha, along with the video and film.

The news will be enthusiastically greeted by protesters who have long complained of intrusive police surveillance of demonstrators and the over-use of 'kettling', where activists are contained in a certain area for an indefinite period of time.

Police argue the policy can help prevent disorder, but campaigners say it is overused and creates an atmosphere of suspicion.