Anthony Davison was not obliged to tell. But in 2002, having explained the situation to family and friends, informing work seemed to be the next logical step. "I was honest with my employer, so if in future there were any issues, I couldn't be accused of springing this on them," says the 33-year-old New Zealander.

A few days after telling the head of HR and his general manager, who were very sympathetic, Davison was informed that, after taking legal advice, there were concerns about the danger he posed to colleagues and the "legal obligation" this put on the company. He was asked to step down as a first aider; it was mooted that he wear a special identification bracelet or carry a card in his wallet; and it was even suggested at one point that a memo would be sent "effectively to all staff" in the London office.

Davison, who is HIV positive, works at the London office of Westpac and was the first of the Australasian bank's 30,000 staff to disclose his status. Despite being ranked the number one bank in the world on corporate and social responsibility, Westpac had no HIV policy.

"The company was doing this from a liability perspective without any reality check," says Davison, who felt bewildered and betrayed by an organisation he had worked for since 1994. "At times it felt that they were casting me aside - as a sort of leper - and that is why I hit back as hard as I did."

Within two week's of its initial "knee-jerk" reaction, the bank came back with some research and a draft policy that guaranteed that HIV-status disclosures would be protected. "It was very much treated as a learning experience," says Davison. "I like to put a positive spin on it, but at the time it was handled very poorly. The experience enforces why, even with the DDA in place, companies need to have an internal policy."

The DDA is the Disability Discrimination Act. In 2005 it made it unlawful to discriminate against people with HIV, from the point of diagnosis. This means that those living with HIV cannot be harassed or discriminated against in recruitment; in employment terms and conditions; in chances for promotion, transfer, training or other benefits; through unfair dismissal or less favourable treatment than other workers, says the Disability Rights Commission.

Also under the Act, employers, if requested, have to make "reasonable adjustments" to the workplace or working practices so that people living with HIV - or other disabilities - are not put at a "substantial" disadvantage. "There have been several cases in which people living with HIV have been told they could not leave work for hospital appointments," says Roger Pebody at Terence Higgins Trust (THT). "The situation is improving - but it is a very slow process, and both employers and employees are often hazy about precisely what the law covers and what counts as discrimination."

In the past year THT Direct gave advice on employment discrimination to more than 500 HIV-positive people - and many more called the helpline to discuss it informally or as part of a wider discussion.

More than a quarter of a century since the first Aids diagnoses in the UK, ignorance surrounding HIV continues. The most frequent form of discrimination in the workplace is an employer breaching confidentiality of HIV disclosure - problematic as there are still hostile attitudes toward HIV. "For the DDA to work, people living with HIV need to disclose their status, but quite reasonably an awful lot are not confident," says Pebody. "Many would prefer to hide things to some extent, taking off days using their annual leave when necessary."

And there is the rub. To access their rights under the DDA, the 65,000-plus people living with HIV in the UK, have to disclose their HIV status to their employer. But few do - 31% of white gay men and 16% of non-white gay men, according to new research conducted among the gay community (there are no figures for HIV-positive heterosexuals) by Jonathan Elford at City University, London.

Despite advances in treatment, HIV remains a highly stigmatised illness. Until the introduction of the retroviral treatment in the mid-1990s, those diagnosed with HIV had a very poor medical prognosis. The effectiveness of retrovirals has seen the number of Aids-related death rates fall from 1,726 in 1995 to 419 last year, according to the Health Protection Agency.

There are side effects, but drugs are allowing people to work and lead long productive lives. "This is still quite recent though in the scheme of things, in the scheme of people's understanding, and in the way that people's attitudes change over time," says Yusef Azad, director of policy and campaigns at the National Aids Trust (NAT). "So we do have work to do educating people."

But attitudes do seem to be changing. A Mori poll in 2005 found 66% of UK adults agreed most people with HIV can work like anyone else, while a majority of respondents said they would be comfortable working alongside someone with HIV.

"So I think an important message to give people with HIV is that the environment isn't quite as hostile as they think it is," says Azad. "Having said that, you only need one person or one employer who's a bastard to cause you a lot of upset."

But HIV-positive people may still be denying themselves the rights that other people normally enjoy because of their fear of repercussions if their status becomes public. "Because of that high degree of self-censorship and non-disclosure in the workplace, people with HIV deny themselves rights that the DDA has brought in," says Azad.

So far, only the case of Scott Donnelly (formerly Watts) has reached an employment tribunal under the DDA. The trained nurse was dismissed in 2004 from his job at High Quality Lifestyle care home in Dover after he disclosed his status. Supported by Ensuring Positive Futures (EPF), an employment programme for those with HIV, Donnelly won his appeal for unfair dismissal.

Yet the very public nature of legal process means that people living with HIV do not take matters that far. Even so, says Andrew Little, EPF programme director, the strength of the law "sends a clear signal - don't discriminate" .

Last month, Anthony Davison was one of four HIV-positive speakers before an audience of MPs, peers and trade unionists at the London launch of the EPF's website. But Davison says the approach of unions such as Unison, which puts the onus on employers, is blinkered. "Employers can't do anything until people do disclose [their HIV status] and, as far as I'm aware, most employers will look after staff with HIV and they have rights under the law," he says.

The National Aids Trust (NAT) has launched the HIV and Recruitment guide for employers and job applicants living with HIV, to help ensure the recruitment process is free from discrimination and unfair treatment. www.nat.org.uk; www.ukcoalition.org; www.tht.org.uk