In an escalation of the intimidation against Huntingdon Life Sciences (HLS), the country's largest daycare provider, Leapfrog Day Nurseries, confirmed yesterday that it had been warned to sever all links with the group.
Letters sent to the home addresses of company directors threatened that the childcare company, which has 10,000 places nationwide, would "suffer the consequences" if it did not withdraw a vouchers scheme offered to HLS employees within seven days.
Leapfrog said last night that it had decided to end the vouchers scheme immediately because of the threat from the Animal Rights Militia.
A spokesman for the nursery company said: "Whilst threats of any sort are totally unacceptable, we have to take them seriously. The care of the children and our staff is of paramount concern. Our business is childcare and we have to take every precaution when it comes to the security and safety of these children and our employees."
Anti-vivisection militants, who have led a long campaign against Cambridgeshire-based HLS, have stepped up their operations in the last month.
The Animal Liberation Front (ALF) this week claimed responsibility for a firebomb left outside the Buckinghamshire home of Paul Blackburn, a senior executive with GlaxoSmithKline (GSK), the pharmaceutical giant, which has links with HLS.
The device exploded on the porch of the house while Mr Blackburn's wife and daughter were at home
An incendiary device was also placed at a sports pavilion in Oxford owned by Corpus Christi College as a protest against a primate laboratory being built by the university. The bomb failed to properly explode.
But the threat against a company caring for children represents a shift in tactics.
Leapfrog was named in a list of 17 companies posted five days ago on Biteback, an American-based website used by activists to publicise their activities.
HLS recently obtained an injunction banning campaigners from targeting any of its sites and militants claimed they were circumventing the ban by focusing on suppliers. But the website message naming Leapfrog said: "We checked in the injunction and can see nothing which prohibits the sending of letters promising to come round and smash the directors' houses up unless they cut all links ... One last thing - this is not a threat but a promise."
While the number of incidents by anti-vivisectionists has decreased, experts have expressed concern that the severity of the attacks is intensifying.
The Association of the British Pharmaceutical Industry warned last month of death or serious injury if the activists were not halted.