Mainstream anti-vivisection campaigners say that their image has been "severely damaged" by the extremist activities of the Animal Liberation Front (ALF) which is backing his hunger strike. They are angry that the ALF "did not tell them" of the animal activist's decision to fast, and say its members failed to share details of the publicity campaign organised around Horne's action.
Yesterday Horne, who was in the 67th day of his fast, saw his family in prison. Les Stevens, a spokesman for the Animals Betrayed Coalition, which is supporting Horne, said the convicted arsonist's health was deteriorating badly: "He saw his family today and they were having the chance to say goodbye to him."
Hope last week that he might decide to start eating again, because of Government moves to protect animals, is fading.
Mr Horne has already lost most of his sight and is deaf in one ear. His liver and other vital organs are being damaged and his weight has dropped from 14 stone to eight. If he does stop his fast, he may be left with long-term health problems, including internal bleeding and perforation of the stomach. He may never regain his sight.
Two leading animal charities, the British Union for the Abolition of Vivisection (BUAV) and the National Anti-Vivisection Society (NAVS), set up more than a century ago to stop the dissection of live animals in experiments, fear Horne has set back their work. They were behind the high-profile advertisements featuring laboratory experiments using the eyes of white rabbits to test make-up. Last month, after a 12- year campaign supported by MPs and public figures including the actress Helen Mirren, the groups saw a British ban passed on cosmetic tests using animals.
They fear, however, that the public will now view them as "extremists" favouring violence and firebombing to get their message across. They believe their public credibility has been severely damaged by the direct action of the ALF.
The groups, which have about 30,000 members, say that although the profile of animals in labs has been raised by Horne's hunger strike, public sympathy for the anti-vivisection movement has "dropped away" because of the extremist stance of "a tiny minority".
"This is very damaging to the animal rights movement as a whole. We are afraid we'll be tarred by the same brush," said one anti-vivisection campaigner. "It is doing us no favours at all. We have spent years getting to where we are today."
The hard-line Animal Rights Militia has threatened to "assassinate" 10 people if Horne, serving an 18-year sentence for a firebombing campaign, dies. The ALF has claimed that senior scientists at the Roslin Institute, near Edinburgh, where the sheep, Dolly, was cloned, could be murdered by Militia members. Last August, the ALF vandalised the cars of four scientists connected to Oxford University, and sprayed on one drive the words "murderous scum". Police have warned other scientists who may be targeted.
A BUAV spokeswoman said: "We want to make sure that the Government does stick to its pre-election promises. But we still feel we have achieved far more under this Labour government than we ever did under 18 years of the Tories."
Earlier this month, the NAVS, which was founded in 1875, persuaded the Home Office to end blanket confidentiality surrounding animal experiments. A spokeswoman for NAVS said: "We don't take the direct action route because we find it's more effective to lobby MPs and use public education campaigns."
Horne has signed a living will that rules out his force-feeding, and made Alison Lawson, a 29-year-old animal rights protester, his next of kin, although he has an older brother and sister and two teenage children. Ms Lawson lives in a Coventry council block known as "Animal Towers" for being the home of a number of animal supporters. The movement's first martyr, Jill Phipps, who died in 1995 protesting against veal- calf exports, also once lived there.
An ALF spokesman said of Horne: "He's very cold and having trouble concentrating. We need to see him, but they've restricted prison visiting times."