SITTING in front of the interview panel, John O'Neill knew it was his big chance to impress. A long sought-after place in the ambulance service was within his grasp, if only he could maintain his composure.

But then the first question arrived and all John's careful preparation went out of the window as the spectre that had haunted him throughout his childhood struck with a vengeance.

"They asked me why I wanted to join the ambulance service. I was sitting there and stammering for about five or six minutes and getting more and more frustrated but I couldn't get any words out," he says.

"My speech fell apart and I thought that was it. It was a job I really wanted to do but I thought I had blown it."

However, instead of trying to interrupt him or speak for him, the interviewer waited until John's stammer came under control and allowed him to finish. Despite his fears, the then 21-year-old was offered the job, but it was a reminder of how his speech impediment had dogged his life.

"I stammered ever since I was really young. From my earliest memories of speaking I always had a stammer. My primary school was not too bad because my friends were very understanding but as I moved up into senior school it was more difficult," says John, who lives in Hartlepool and is now a trainee technician for the Tees, East and North Yorkshire Ambulance Service.

"Kids are quite cruel and I took a fair bit of stick. They would pretend to be stammering all the time and, if I was asked a question in class and I stammered, they would laugh. As soon as I knew I was going to be asked a question I got a massive shake going through my system because you need to be getting up and speaking. You so much want to speak clearly and fluently but you can't. As soon as your speech fell apart they would laugh, you felt embarrassed and not part of the group."

Experts are still puzzled over why some people stammer, putting it down to a combination of factors, and the severity varies widely between sufferers. Some stammerers struggle with particular words or sounds, which they can learn to avoid, but for others, including John, it could strike at any time.

"At certain times, I couldn't say my own name, I was stuck on the first sound. On really bad, bad days it was all words."

Although his teachers were understanding, and tried not to ask him to stand up in class, they were conscious of the danger of singling him out for special treatment and making him feel even more excluded. And when he left school, and went on to Hartlepool Sixth Form College, the problem showed no sign of abating.

"It was something I lived with all my life but, as I got older, I had to think about it a lot more. The time I spent at college was really good but my speech was very poor.

"I had tried speech therapy from the age of six and that had no effect. I had been seeing a psychologist since I was 16 and, when I was 20, I went to a hypnotherapist but that had no effect."

After leaving college, John went to work in a shop and from there applied to join the ambulance service. But, even after surviving the ordeal of his interview, things were still difficult.

"Certain days were really hard going. Using the radio on the vehicles was hard work, speaking to patients was hard, so was speaking to nurses in hospitals."

About one per cent of the population stammer, with three or four times as many men as women suffering, according to speech and language therapist Anne Ayre.

"Stammering is very complex and we still don't really know what causes it. We know it is a mixture of physiological and linguistic and environmental factors, and one of the things that makes it difficult for stammerers is that there is no specific cause," she says. "And how much you see on the outside often bears no relation to the overall impact the stammer has on a person's life. Sometimes, somebody can appear to have a very mild stammer but their feelings of shame and embarrassment and fear can be overwhelming.

"Anything where verbal communication is key may be a problem and people can often suffer from low self-esteem and feel quite isolated. You can also get people who are covert stammerers, who have successfully hidden all the external factors, sometimes even hidden it from their family. They spend their whole life trying to ensure that nobody finds out that they stammer, but the stress is enormous."

Anne, who lives near Darlington, and is in the final stages of a PhD looking at stammering in the workplace, says stammerers often rule themselves out of jobs where they fear they will be unable to cope, in addition to the problems they face with employers. "Some people may incorrectly perceive people who stammer as not being as competent as their peers, they see them as being more nervous and perhaps less intelligent. Employers often have negative attitudes and they can be a considerable barrier to people who stammer, both in getting employment and in succeeding once they're in work.

"Quite often, people who stammer may be working below their capabilities, sometimes they're limiting themselves and they think if they get promotion they will have to make presentations and they would not be able to do this.

"If an employer is aware they have an employee who stammers, I believe very strongly that they have a responsibility to raise awareness in the workplace and aim to provide a supportive environment where a person who stammers can contribute fully and realise their potential. Not everybody has problems, some people cope absolutely brilliantly, but for others it is very stressful."

And for people who do have problems, a change in workplace culture to put more emphasis on communication and team-building, as well as an increasing number of jobs which rely on the telephone, can make the outlook even bleaker.

But some employers are recognising the difficulties. As part of Anne's research, she worked with British Aerospace, who were concerned that some employees were not reaching their potential. Sessions highlighted ways to help stammerers contribute in meetings, including preventing them from being interrupted, and making people aware of the problems.

And for some sufferers, there is hope that stammering is not a lifelong curse. In January 1999, the ambulance service paid for John O'Neill to undergo the three-and-a-half day McGuire programme. The intensive course teaches stammerers to breathe from their diaphragm instead of their stomach, producing a smooth, uninterrupted flow of air and helping them control their speech. Added to sessions on the psychological and emotional factors, it had a remarkable effect on John, now 24.

"I was fortunate and I got the technique straight away. For the first three or four hours on the course you don't actually speak at all and then they ask you to say your name.

"Pretty much everyone said their name for the first time in ages. It was unbelievable. I can't describe it, it is just a great feeling. I will never feel that good again."

John describes himself as a recovering stammerer and still has some problems, including one major relapse, but now he knows what to do and how to bring it under control.

"I stammered all my life and after three-and-a-half days on this course I was speaking fluently. It was a complete turnaround, my confidence was sky high and if you speak to my colleagues they say I never shut up. I haven't shut up ever since."

* The British Stammering Association can be contacted on (0208) 981 8818.