The Sunday Telegraph can disclose that Mr Blunkett has discreetly served as a patron of the Humane Research Trust, which aims to eliminate the use of animals in human medical research.

It is understood, however, that his name was removed from the list of patrons after he became Home Secretary three years ago because the charity feared that it would cause him embarrassment.

He is said, nevertheless, to continue to support the group and its peaceful aims.

Last night Mr Blunkett was accused of having a conflict of interests and there were calls for him to resign.

Under the Ministerial Code, ministers "must ensure that no conflict arises, or appears to arise, between their public duties and their private interests, financial or otherwise".

Aisling Burnand, the chief executive of the BioIndustry Association, whose members have been targeted by animal rights activists because of their involvement with legally-required animal experiments to test drugs, said "This explains a lot: why the Home Secretary has been silent on this issue.

"It is astonishing that the Home Secretary, who is meant to defend law and order, ignores the plight of people working in or with bio-medical research.

"We have had people firebombed, hit with baseball bats and their cars sprayed with paint and acid. David Blunkett should examine his conscience as to whether he can remain Home Secretary if he is compromised in this area."

In recent weeks, leading British companies involved in animal research have warned that they will withdraw investment from this country - with the loss of thousands of jobs - unless the Government starts to tackle the problems of violence and intimidation from the Animal Liberation Front and other groups.

Today's disclosure has infuriated the pharmaceutical and other industries that use animals for research. They have repeatedly complained that unlike Jack Straw, the former home secretary, Mr Blunkett has refused to confront illegal activists.

On Friday, a Sunday Telegraph reporter introducing himself only as a "member of the public" telephoned the charity and asked for a list of the patrons. An official from the charity, who did not give her name, read out their 12 names.

At the end of the list, she was asked whether Mr Blunkett, too, was a patron. She replied: "He is, but we don't put his name on any more because [of], obviously, the job he is in at the moment. Privately he does support us but not publicly, as it were."

Asked again to confirm that Mr Blunkett remains a patron, she said: "Not really a patron. He is one of our supporters now rather than a patron. The job he is doing doesn't [awkward laughter] lend itself to what we do."

Brian Cass, the managing director of the Cambridgeshire-based Huntingdon Life Sciences (HLS), Europe's largest contract testing laboratory, which has been targeted since 1997 by Stop Huntingdon Animal Cruelty (Shac), said: "This is fascinating and perplexing.

"The Home Secretary has been unwilling to engage on this issue [animal rights] and to put in place protective measures for those who are being illegally and persistently harassed and intimidated."

Earlier this year The Sunday Telegraph revealed that HLS is considering private prosecutions against Shac's leaders because the Government has failed to charge them with alleged offences.

Mr Cass wrote to Mr Blunkett last week urging him to make a public statement condemning animal rights activists. HLS says that a ministerial committee looking at the illegal activities of animal rights activists - and chaired by Mr Blunkett - has come up with no effective measures.

Mr Straw, however, was considered by HLS to be firm and courageous in his attempts to thwart the activists. He visited the company at its headquarters in April 2001, and said: "We will not tolerate a small number of criminals trying to threaten research organisations and companies."

The Sunday Telegraph could find no record of his public views on vivisection or on animal rights protesters.

A spokesman for the Home Office said yesterday: "Mr Blunkett was a supporter of the Humane Research Trust in the early 1990s.

He was supportive of what they were doing: trying to find alternatives [to animal testing].

" The spokesman said that Mr Blunkett "probably" never formally severed links with the charity and she refused to say what his current views are on vivisection.

"It is completely ridiculous to suggest this support [in the 1990s] would have influenced government policy or his stance.

"He believes Huntingdon Life Sciences has a role to play and he would condemn any act of violence by any anti-vivisection group. There is ongoing work to tackle the violence by animal rights activists."