GlaxoSmithKline has mounted a legal offensive against unnamed animal rights activists that sent threatening letters to some of its shareholders, winning a high court injunction against the group.

The injunction will make it illegal for the activists to publish the personal details of investors in GSK on the Web – potentially laying them open to reprisals – if they refuse to accede to demands to sell their stakes in GSK. Any person found in breach of the injunction could face prosecution for contempt of court which is an imprisonable offence.

The UK headquartered company said yesterday that the investors had been given two weeks to sell their shares or face the consequences. The demands have been made because of GSK’s continued use of services provided by Huntingdon Life Sciences, a perennial target of animal rights groups.

The mailing on May 1 is the latest installment in a long-running and occasionally violent battle between the pharmaceutical sector and those who believe animal testing of any type is wrong and should be banned.

In one of the worst incidents, HLS managing director Brian Cass was beaten with pickaxe handles by three masked attackers in 2001. A man was subsequently jailed for three years in connection with the incident.

And last July, a broker working for Canaccord Capital, which held an interest in another HLS customer, UK drugmaker Phytopharm, escaped injury when a bomb placed underneath his car exploded. A group affiliated to the Animal Liberation Front claimed responsibility for the latter attack.

“This letter campaign is a typical tactic used by these extremists and is intended to cause fear and intimidation,” said GSK. “This injunction supports those efforts being made by the police, who are taking this matter very seriously, and are conducting a criminal investigation into these matters.”