EMPLOYEES of a pharmaceutical company have been warned to be on their guard against animal rights extremists.

The warning for the 1,150 staff at GlaxoSmithKline's (GSK) Barnard Castle site, in County Durham, comes after extremists launched a campaign against its shareholders.

The extremist group has targeted GSK over its continued use of medical research group, Huntingdon Life Sciences.

Letters had been sent to shareholders around the country urging them to sell GSK shares to target its financial vulnerability.

The company said about 50 private shareholders had contacted the company to complain about the letter, although it suspected many more had been targeted.

In a letter dated May 1, the extremists warned shareholders: "Should you choose not to sell your shares within the next 14 days your details will be published and within weeks a website will be hosted with all remaining shareholders listed."

GSK said the letter campaign is a typical tactic used by extremist groups and is intended to cause fear and intimidation.

The company has urged anyone who receives the letter to inform the police, who have launched an investigation.

A spokesman at GSK's Barnard Castle site said: "We have sent an e-mail round to all our employees and put copies up on notice boards.

"The e-mail confirms the fact that there has been action by an extremist group and advises employees what to do if they receive a letter."