It is, as Tony Blair admits, extremely unusual for a serving prime minister to add his name to a public petition. Most such demands, after all, are directed at the Government, and it cuts across political logic for the Prime Minister to send a petition to himself. But Mr Blair’s decision to sign the on-line declaration of support for animal testing is intended to underline his personal commitment, as well as that of the Government, to opposing animal rights terrorists, the “tiny group of extremists” threatening medical research in Britain.

That commitment is long overdue. For despite legislation increasing the penalties for economic sabotage and promises to give better protection to laboratories and personnel engaged in animal testing, the threat from extremists has not diminished. The absurd lengths to which some fanatics are prepared to go was underlined last week at the trial of the four people found guilty of desecrating the grave of a woman whose family runs a farm breeding guinea-pigs for research. The judge handed down long jail sentences to reflect public disgust at this obscenely macabre crime and also to deter other extremists. But that has done little to stop the harassment of companies, individuals and even the families of those engaged in animal testing.

The latest attempt at intimidation is the threat to investors in GlaxoSmithKline (GSK). Anonymous letters warned them that if they did not sell their shares in the large pharmaceutical company, their names would be placed on a website. The clear implication was that they would then become targets for the kind of harassment suffered for years by employees of Huntingdon Life Sciences. GSK has won a High Court injunction against such action. But the threat has already had an inhibiting effect, and the wariness of international companies, especially Americans, in investing in medical research in Britain is a serious setback to an industry that is as important as it is successful.

Mr Blair rightly underlined the huge contribution British research laboratories have made to the advances in the treatment of cancer, muscular dystrophy and Alzheimer’s disease. They deserve, as he declared, thanks, support and protection. Last year’s measures intended to counter the terrorism of animal rights activists — violent nihilists who have hijacked a cause to justify their own pathological behaviour — were important. But the police and courts must keep up the pressure until not only violence is halted but all intimidation is stopped so that laboratories can carry out their pioneering work.

The Times has long argued that the needless suffering of animals should be banned. Animal experimentation must be strictly regulated, and wherever possible alternatives such as computer modelling should be used. This is also the position now taken by the Government. Testing on great apes is not permitted, and in 1998 the Government outlawed the testing of cosmetics on animals. No animal procedures are allowed unless an independent panel can first be convinced that it is necessary.

With these safeguards in place, however, it is essential that vital tests on possible scientific breakthroughs go ahead. Those who, purporting to defend animals, would deny the chance of life or recovery to millions suffering from debilitating disease threaten not simply our laboratories; they threaten our very humanity.