The act was carried out as part of an increasingly violent campaign against the Huntingdon Life Sciences laboratory in Cambridgeshire, which GSK are known to have close links with.
Despite recent government legislation designed to protect workers of, and companies linked to animal research facilities, this latest episode threatens to give back animal activists the upper hand in determining the future of animal experimentation within the UK.
According to details published in the >Guardian newspaper, an anonymous animal rights group claimed to be writing to all 170,000 small investors warning them to sell up. However, it is unclear how many did receive a letter.
The letter stated that animal rights activists would "hold Huntingdon Life Sciences accountable for its acts of animal cruelty" and warned that every shareholder would receive the letter in the next two weeks."
The letter went on to read: "The only way to hold GlaxoSmithKline to it's [sic] PROMISE is to target it's [sic] financial vulnerability. We are therefore giving you this opportunity to sell your shares in GlaxoSmithKline."
"If you have any doubts over the effectiveness of this action then keep a close eye on the GSK share price and watch it plummet."
The letter's recipients were then requested to sell their shares within 14 days and inform the group via email.
The letter ended: "We will be checking that you have done this."
"The choice is yours."
GSK issued a statement and advice instructing shareholders on action to be taken should they receive one of these letters. The statement also reassured shareholders that the police had been informed and an investigation had been started.
The pharmaceutical company said: "We are very concerned that individuals have been targeted in this way, and condemn the group responsible."
"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. Animals are only used where no alternative is available and our scientists constantly strive to reduce the numbers used. We aim to exceed industry standards in the care and welfare of the animals it uses."
They emphasisied their support for legitimate animal welfare concerns, and the law abiding organisations that represent them, such as the (RSPCA) Royal Society for the Prevention of Cruelty to Animals.
The Huntingdon Life Sciences research facility has had to bore the brunt of a vicious and intimidating campaign of violence, which has seen property on site damaged, and workers threatened.
In addition, companies with business links to the facility have also been targetted. Construction group Montpellier, withdrew from building a laboratory in Oxford to house animals for testing after its share price fell.