· Doubts remain until more tests can be carried out · Millions could benefit if recovery can be explained

A man who claims to be the first in the world whose immune system has been able to beat the HIV virus was facing mounting pressure yesterday to submit to further vital medical tests. Health experts, Aids campaigners and gay rights activists urged Andrew Stimpson to come forward following claims that he has been able to rid his body of the virus after taking little more than vitamins.

Activists say that if the claims are true, the phenomenon could potentially bring countless benefits to millions of people infected with HIV. There are more than 53,000 in the UK alone.

Mr Stimpson, 25, twice tested positive at the Victoria Clinic for Sexual Health in west London in August 2002. A test 14 months later appeared to be negative. But the heath trust concerned, Chelsea and Westminster, yesterday said Mr Stimpson had so far "declined" to undergo further tests. It is understood that he was first asked to do so immediately after last year's negative test result.

A spokeswoman said: "I can confirm that he has a positive and a negative test. I can't confirm that he's shaken it off, that he's been cured. We urge him, for the sake of himself and the HIV community, to come in and get tested.

Though Mr Stimpson insists he will "do anything I can", associates said yesterday that he had gone away to rest and escape the media spotlight.

Campaigners are annoyed that having not yet undergone the vital tests, Mr Stimpson nevertheless signed contracts with the News of The World and the Mail on Sunday, both of which published his claims yesterday.

They also sounded a note of caution, noting that disclosures in his case arose not from medical research or peer review but from legal correspondence relating to an action Mr Stimpson was pursuing against the health trust. He had feared the positive results might have been wrong and had sought compensation. The trust's contention that both sets of blood tests were accurate emerged as it tried to defend itself from litigation.

Mr Stimpson, who lives in London with his partner, who is HIV positive, said: "There are 34.9 million people with HIV globally and I am just one person who managed to control it, to survive from it and to get rid of it from my body. For me that is unbelievable - it is a miracle. I think I'm one of the luckiest people alive.

"I was just taking daily supplements to keep myself as healthy as possible so as not to get full-blown Aids."

But Annabel Kanabus, director of the Aids charity, Avert, said he must match words with deeds. "He must come forward. Organisations such as ours will be inundated this week. There is enough confusion surrounding the issue of HIV. We don't need any more."

She said the sequence of events is troubling. "He was told in October that he would not be paid by the trust so he goes to the newspapers. I think he should have gone straight to his doctors."

Peter Tatchell, of the campaign group Outrage, said Mr Stimpson must "cooperate with the medical authorities for the greater benefit of everyone who has HIV".

Deborah Jack, chief executive of the National Aids Trust, added: "Without further tests it is impossible to draw any conclusions for people living with HIV. The virus is extremely complex and there are many unknowns about how it operates and how people's bodies react to it."

Mr Stimpson, who works in a sandwich bar, moved to London four years ago from Largs in Ayrshire. He took as confirmation of his cure a letter from the NHS Litigation Authority. Edwina Azimi, the authority's case manager, is reported to have denied any false diagnosis, hailing the results as "exceptional and medically remarkable". She said: "You have recovered from a positive antibody result to a negative result." She said there is "particular interest in these rare circumstances".

Whether doctors formally referred the matter to the Health Protection Agency, which monitors the spread of infection throughout the UK, is unclear. Such a referral would be expected to take place in a case of such apparent significance.

Experts stress the potential benefits a genuine case could have for the development of treatments and vaccines. But they also warned that the complexities of HIV make any one of a number of scenarios possible in this case. Tests usually indicate antibodies rather than the virus. They are usually accurate but one of the number of tests he has undergone may have been wrong. In any event a test for the virus itself is more conclusive.

Dr Patrick Dixon, founder of the international Aids agency ASET said a few people have immune systems capable of preventing HIV from becoming established. "It would not be surprising if we find a tiny number of people who have the capability to destroy the infection. It is possible this individual has something special about his genetic code." But he added: "The answer may turn out to be very complex. We must not jump to conclusions."

FAQ: HIV virus

Have there been any documented cases of people beating the HIV virus without medication?

With previous claims of so-called miracle cures, questions have been raised over the testing process or inaccurate labelling of samples. However, there are anomalies. In Kenya, tests on prostitutes showed that their "T cells", which help produce antibodies to HIV, had been exposed to the virus. Yet there was no evidence of HIV in blood or antibody tests

How widespread is HIV? Up to 5m cases are being diagnosed around the world each year. It is estimated that 53,000 people are living with HIV in Britain, and a third of them are yet to be diagnosed

How are people tested?

Initial blood tests err on the side of caution to ensure infected people are not "missed". There are occasionally false positives. First and follow-up tests to confirm diagnosis look for antibodies to HIV, which provide evidence that the body is fighting the virus. A third batch of tests help doctors gauge how far the infection has advanced

How are people treated?

Usually using a combination of antiretroviral drugs, although there is growing concern about the virus adapting to resist them