Some of GlaxoSmithKline’s biggest shareholders have received threatening letters from an animal rights organisation, demanding that they sell their holdings in the firm or face the consequences.

Although the organisation behind the letters is unknown, GSK has been a target for those who oppose animal experimentation, both for its own activities in drug development and as a customer of Huntingdon Life Sciences, which was forced to relocate its main operations to the USA almost four three years ago and de-listed from the London Stock Exchange, saying it needed to protect itself and its shareholders from the actions of animal activists.

The letters - mainly directed at smaller shareholders - said the holdings must be sold within two weeks, or those targeted would have their personal details published on the Internet.

In a statement, GSK said it had informed the police of the letters and offered advice to all shareholders who had received one. It also offered them the chance to transfer their shares into an anonymous holding.

“For ethical, regulatory and scientific reasons, research using animals remains a small but vital part of the research and development of new medicines and vaccines,” said the company. “Animals are only used where no alternative is available.”

The company also reiterated its support for Huntingdon, “as long as they maintain their current high standards of animal welfare in line with Home Office requirements.”

One group running a campaign against the company, Stop Huntingdon Animal Cruelty (SHAC), lists GSK as being among its ‘targets’, maintaining that the UK company is Huntingdon’s single largest customer.

Last year, in defense of companies who test on animals, the UK Government passed new laws making it a criminal offence under the Serious Organised Crime and Police Act to target any scientist, research facility or company in the supply chain with a campaign of unlawful acts, which includes criminal damage, trespass, blackmail and libel. Such offences carry a penalty of up to five years in prison and an unlimited fine.

The new measures caused a halving in attacks involving damage to company, personal and public property in 2005 compared to the prior year, according to the Association of the British Pharmaceutical Industry (ABPI).