Most of us would not hesitate to tell family and close friends about being diagnosed with a serious illness or chronic condition. But telling colleagues is another matter. You might be worried about their perception of your abilities, or simply not feel like sharing that kind of personal information with them. Richard M Cohen, the author of Strong at the Broken Places, which recounts the stories of five people living with chronic illness, kept his diagnosis of multiple sclerosis secret from his colleagues in the television industry, where he worked as a news producer, for almost a decade. He recalls a time when he went for a job interview and asked a friend if he thought he should disclose his condition. The friend responded: "Say nothing." Cohen agreed, and got the job. Years later, after his condition was known, the person who employed him said he was right to keep quiet. "I am not proud to say this," Cohen recalls the man saying, "but I don't think I would have hired you if I had known."

Mike Wilkins, a former businessman who was diagnosed with MS 20 years ago and who is now a volunteer campaigner for the MS Society, understands the reluctance to share a diagnosis. "At the time I was running various companies," he says, "and I had in the region of 10 to 15 people working for me. It was a great shock to the system. I was concerned for myself and for my family but also for the people working for me. I did feel that pressure." He kept his condition secret from his employees and business associates for several years. "Certainly during those first few years I was in a form of denial. I didn't want to show any weakness or areas I could be exploited in. I was concerned for my business interests, because it can be a cut-throat world."

Of course, with serious illnesses that will require extended periods off work, you may have little choice but to tell some people something. But revealing details of your new diagnosis to everyone in your company is still something that many people, unsurprisingly, shy away from. Annabel Wright, an office worker for her local council who is being treated for breast cancer, says: "Initially I just wanted to tell the bare minimum of people. At the time, you're going through all the tests and you don't know the outcome. The doctors won't tell you what it is until they're sure and I didn't want lots of people knowing and asking me about it, because it was a difficult time." Once she had gone on sick leave, Wright asked two colleagues to pass on the news to the rest of her workmates, but still acknowledges that it is difficult when a diagnosis becomes common knowledge. "People get very upset while you're trying to stay positive," she says. "And they ask when you're going to be at the hospital and phone afterwards. It's well-meaning, but it can be very draining."

Ayesha Owusu-Barnaby, head of campaigns and public affairs at Macmillan Cancer Support, says that working with a serious illness is something that is going to affect more people over time. "Because of demographic changes and changes to the retirement age, we are going to see more people being diagnosed while still in employment," she says. "And it's not just cancer - people are living longer, and living with a variety of conditions." She also points out that while it's ultimately the patient's decision as to how much they want to reveal, it will make things easier if they decide on a strategy for disclosing at least some information. "We'd say start by telling someone at work who you trust," she says, "and decide with them how you will manage the process. People will notice if you have an illness that will mean you have to make changes at work, and it may be easier to give people the facts. But once you've done that, it's OK to say: 'I don't want to talk about it', or, equally, 'I am happy to talk about it'."

Macmillan has produced a guide, available at macmillan.org.uk, for employees managing the effects of an illness. It also launched a campaign last year called Working Through Cancer to improve the advice and support that people get in the workplace. "Managers are going to have to get to grips with how to deal with employees who have a serious illness," says Owusu-Barnaby. "Our research suggests that most employers do want to do the right thing, but there currently just isn't the right kind of support and advice available at work."

This was certainly Wright's experience after her own diagnosis, as she was given conflicting information and advice about her rights to sick leave and pay from different people within her company. "I had to wonder: does anyone really know how to handle someone in my situation?" she says. "And the answer I unfortunately came up with was no. They had the best will in the world, but they weren't trained for it." And when you're struggling to come to terms with the news that you have a serious illness, tangling with HR and your line manager is the last thing you feel equipped to do. "You would think that in a large organisation there would be someone who is clued up about the law and who has an emotional understanding of what we're going through," she says.

Sarah Veale, head of equality and employment rights at the TUC, acknowledges that it is difficult territory. "It is complicated and many employers do struggle to make sense of it themselves," she says. Under the Disability Discrimination Act, the employers of people suffering from many forms of illness, chronic conditions or disabilities have a duty to make reasonable changes to the working environment to enable the individual in question to do their job. "What good employers should do if they're not sure of the law is get advice," she says. And the same goes for patients. "There is a lot of good information out there that is free, authoritative and easy to access," she says. Good places to start are the websites of the Department for Work and Pensions, the Equality and Human Rights Commission (which has absorbed the Disability Rights Commission) and Acas. She points out that there should never be any need for illness to cause severe difficulties between you and your employer and that, in most cases, employers are eager to help.

Wilkins says news of his MS gradually became widely known. "You know what it's like," he says. "I told one person who told one person ... and, before I realised it, most of my business associates were aware." And he does not regret that his secret came out. "At the time I was very anxious about what the future would hold, but I have found that people are very supportive and willing to try to understand. I can now see the benefits of being open, and if I did it again I would take a different route from the start. When I was diagnosed, I wanted to curl up into a ball, but if you learn to reach out you can embrace all the help and fellowship that is there for you."