Writing in the Sunday Telegraph, he said companies with links to animal testing may be allowed to keep details of their shareholders secret.

Mr Blair said he would also sign a pro-animal testing petition in a stand against a "tiny group of extremists".

He said threats against GlaxoSmithKline shareholders showed why those in medical research had to be protected.

Mr Blair's support has been widely welcomed by the science community.

But other animal rights activists have criticised the prime minister's stance.

'Life-saving research'

Drugs giant GlaxoSmithKline was targeted because of its work with the medical research laboratory Huntingdon Life Sciences.

Mr Blair wrote: "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.

He also said he would sign the People's Petition in support of animal testing in the UK, saying he wanted people to "stand up against the tiny group of extremists threatening medical research and advances in this country".

But Alistair Currie, Campaign Director for the British Union for the Abolition of Vivisection, said the prime minister was "backing the wrong horse".

"The reality is that animal experimentation is old-fashioned science - it may have been useful 50 years ago but in the 21st century we have a lot of techniques that we can use instead," he said.

'Benefit of society'

Seven big investment firms have voiced support for GlaxoSmithKline (GSK).

In a letter to the Financial Times, the firms - each with billions invested in the health sector - pledged to maintain their stake in the drug group.

Police are investigating the anonymous letters from the previously unknown Campaign Against Huntingdon Life Sciences, sent to at least 50 shareholders.

They threaten to reveal investors' names and addresses on the internet if they did not sell their shares within a fortnight.

GSK obtained an injunction earlier this week against the campaigners.

It 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.

In the letter to the FT, the funds said they would not allow animal rights extremists to bully them into selling their substantial shareholdings in firms "legitimately engaged in lawful research to the benefit of society".

The letter to the newspaper said: "Whilst we understand the genuine concerns of animal welfare groups and support moves to reduce, refine and replace the use of animals in medicines research, there is no place for intimidation and violence in this process."

Scientists' support

Lord Robert Winston, of Imperial College London, said: "I am delighted that the prime minister is strongly supporting an endeavour that is entirely humane, ethical and legal and vitally important to the health and wellbeing of people all over the world."

Professor Colin Blakemore, chief executive of the Medical Research Council and himself a victim of animal rights activists, said: "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 British Heart Foundation's professor of cardiovascular science, John Martin, said it had been "a lonely experience against a powerful lobby involving very few fanatics" when he started conducting heart disease research using animals.

But NHS patients had gradually voiced their public support of such research.

"This new popular movement has culminated with the prime minister himself, who has been a patient, supporting the cause of science and sanity," he said.