LONDON (Reuters) - GlaxoSmithKline Plc secured a rare high court injunction on Tuesday night against an unknown group of animal rights activists, preventing them from publicising names of its shareholders.

The move makes illegal any attempt by campaigners to carry out a threat to publish on a Web site the names of private investors who refuse to sell their shares in Europe's biggest drugmaker, a company spokeswoman said.

It is the first time such an injunction has been granted to a company in Britain.

Scores of small shareholders have this week received menacing, unsigned letters demanding that they sell their shares within the next 14 days because of Glaxo's continued use of animal research laboratory Huntingdon Life Sciences (HLS).

Glaxo said the police were treating the matter seriously and had started an investigation.

The mailing follows a prolonged -- and sometimes violent -- campaign by British animal rights groups who demand drug companies end all animal research.

The pharmaceuticals industry argues that using animals remains a vital part of the research and development of new medicines and vaccines.

Many of the world's top 100 medicines were discovered in Britain, which is home to industry giants such as Glaxo and AstraZeneca Plc, although the country accounts for less than 4 percent of the world drugs market.

HLS has been the target for the most vociferous campaigning and intimidation by animal rights activists over the years, eventually leading the company to quit the London stock market.

Glaxo's continued involvement with HLS is regularly condemned by protesters at the company's annual general meeting, which will be held this year on May 17.

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