Letters sent by a group calling themselves the Animal Rights Militia (ARM) to the home addresses of directors said "your family is a target" and they would "suffer the consequences" if they did not comply with a two-week deadline to sever all links.
It emerged last week that directors of Leapfrog Day Nurseries had received similar letters. The company had offered childcare vouchers to staff at HLS - something they have now stopped. Now nine companies - more than half of those targeted - have severed links with HLS.
Many of the firms targeted did not even work directly for HLS - one had simply collected three lorry-loads of rubble from a construction site. Most of the companies are small-scale building contractors in the Peterborough, Huntingdon and Harrogate areas.
The letters, sent to the homes of directors at 17 companies, said: "The company you work for is working with Huntingdon Life Sciences. This is a disgusting and cowardly act. You have a choice. You can walk away from those sick monsters or you can personally face the consequences of your decision. Not only you but your family is a target. Sever your links with HLS within two weeks or get ready for your life and the lives of those you love to become a living hell."
The letters also referred to the "victory" against the Hall family at Darley Oaks Farm in Newchurch, Staffordshire, who were targeted by animal rights extremists for breeding guinea pigs for medical research. During a six-year campaign, the remains of a family relative, Gladys Hammond, were dug up and stolen. They have never been retrieved. The family announced in August that they would no longer breed guinea pigs.
More companies may decide to sever their ties with HLS this week. Ian Bailes, a director of Most Construction in Harrogate, which has carried out some building work for HLS, said the firm would be making a decision shortly.
"The tactics have changed," Mr Bailes said. "Vocal protests are fine ... That is not a problem. It is the militant ones that are a problem. The police need to wipe them out."
It is a view echoed by the organisation co-ordinating the police response to animal rights extremism, the National Extremism Tactical Co-ordination Unit. "A lot of people think they target big pharmaceutical companies," a spokesman said, "but they go after companies with any link, however tenuous."
Police believe the number of activists engaged in criminal activity is relatively small. "There are perhaps 10 to 15 full-time organisers identifying targets," a police source said. "Then there is a wider network of 40 to 50 activists who are prepared to commit the crimes. Beyond that there are up to 600 to 700 supporters maximum. They know what is going on and turn a blind eye."
The main campaign against HLS has been run by Stop Huntingdon Animal Cruelty. Greg Avery, Shac's founder, welcomed the companies' withdrawal but denied his organisation had anything to do with the threatening letters sent by ARM. "These letters are not something we condone or support," he said. "It is something we have condemned hundreds of times." But he warned that "lots of activists are really, really angry", and predicted that people could get more violent.
Brian Cass, managing director of HLS, accused Shac of orchestrating the violence. "Companies are named by Shac and then the trouble begins. Shac, Animal Rights Militia, Animal Liberation Front are to all intents and purposes the same thing."
A police spokesman pointed to the example of Sarah Gisbourne, a Shac activist serving a six-and-a-half-year sentence for attacking property connected to HLS. "Based on historical experience, they can be seen as one and the same," he said.Reuse content