Anti-vivisectionists have criticised Tony Blair's pledge to sign an online petition which backs animal testing.
GSK works with the medical research lab Huntingdon Life Sciences
The prime minister, who condemned the "appalling" actions of animal rights extremists, will join around 13,000 people on the People's Petition.
He said threats against GlaxoSmithKline shareholders showed why those in medical research had to be protected.
However, the National Anti-Vivisection Society described Mr Blair's commitment as "hugely irresponsible".
Jan Creamer, the society's chief executive, said: "We understand this petition has only 13,000 names.
"This is compared to over 20 times that number of people who support animal welfare groups on non-animal research, plus the overwhelming public support for replacement of animals in testing.
"This petition is being run by an extremist group of vested interests representing a very narrow area of medical research.
"They want to see the UK continue with an outdated method of research as opposed to taking up more advanced, non-animal scientific methods."
The British Union for the Abolition of Vivisection praised Mr Blair's tough stance against extremists but urged him to find alternatives to testing.
Campaign director Alistair Currie said: "Tony Blair seems to be blindly backing the animal experimenters in the mistaken view that what's needed is solidarity with them.
"What's actually needed is a cool and objective look at the profound animal suffering and outmoded science that animal experiments represent."
Mr Currie added that animal experiments had begun to rise again since Labour came to power in 1997.
Conservative MP Roger Gale, president of the Conservative Animal Welfare Group, added: "I fail to see how this gimmick can please or fool anyone.
"He has had the opportunity to instigate a Europe-wide commitment to funding research into the search for validated alternatives to the use of animals in laboratories and he has signally failed to take that opportunity."
However, others were quick to praise the prime minister.
Jean-Pierre Garnier, chief executive officer of GlaxoSmithKline (GSK), said: "I am greatly encouraged by his personal commitment, and that of his government, to ensuring an environment in the UK that is conducive to the research and development of vital new medicines."
Around 50 of the firm's shareholders were sent letters from the Campaign Against Huntington Life Sciences threatening to publish their names on the internet if they did not sell-up.
GSK said it would continue to work with Huntington Life Sciences despite the threats, and urged anyone sent a threatening letter to report it to the police.
Professor Colin Blakemore, chief executive of the Medical Research Council, said: "I know, from my own experience, what it feels like to be a victim of animal rights activists.
"On behalf of medical researchers, who live in fear of such intimidation, I thank Tony Blair for his personal courage in declaring his support for the use of animals in research, when there is no alternative."
The Association of the British Pharmaceutical Industry, the British Heart Foundation and the Coalition for Medical Progress also applauded Mr Blair's personal commitment.
Mr Blair, writing in the Sunday Telegraph, said: "The appalling details of the campaign of intimidation - which include grave-robbing - show the depths to which the animal extremists are prepared to stoop.
"The letter writing campaign just launched against GlaxoSmithKline shareholders shows why we must step up efforts to support and protect individuals and companies engaged in life-saving medical research."
The planned Company Law Reform Bill would offer them more protection, the prime minister said.
Directors would be able to keep their home addresses private and firms could refuse to disclose its shareholders.
Mr Blair said that British scientists and companies made a "huge contribution to human health and well-being", while creating thousands of highly-skilled jobs.
His comments came days after four animal rights activists were jailed in a case related to the illegal exhumation of the body of a woman whose family bred guinea pigs for use in animal experiments.