While I wait for Gareth Nelson to arrive, I worry that he will not like it here. I am in a cafe in a town in north Wales. He suggested it, because he lives nearby, but it is very noisy - babies are screaming and chairs scrape across the floor - and the artificial lighting is quite bright. Nelson has Asperger's syndrome (AS), a disorder (though he would not like me calling it that) on the autism spectrum, and I fear he might get sensory overload in a place like this.

"I don't mind it too much," he says, when he arrives. Sudden movements, apparently, affect him more. I knew it was him. He is dressed all in black and looks young (he is 19) and nervy. When I smile at him, he does not smile back although he seems slightly more friendly when we sit down. His lack of social skills is one negative trait of his AS, which was diagnosed when he was 14, he says, but it does not bother him greatly (like a lot of Aspies, the term many with AS give themselves, he prefers to socialise online). The only other negative characteristic he can think of is his bad organisational skills. "I've never considered the other traits, like being introverted, as a negative," he says.

Nelson, with his wife Amy, who also has AS, is leading the UK's autism rights movement. They run their group, Aspies for Freedom (AFF), from their home; it started as a website three years ago and now has 20,000 members, most of them autistic. AFF came about partly to campaign against the search for a cure. It holds protests - its members turning up with banners - at fundraising events for autism charities.

"I don't think you should cure something that isn't purely negative," says Nelson. "It's the same as black people, who seem to be more at risk of sickle cell disease than white people but you're not going to attempt to cure 'blackness' to cure sickle cell." The idea that the autism rights movement is similar to the civil rights and gay rights movements is something I will hear again and again. Aspies for Freedom even modelled its Autism Pride Day, held on June 18 every year, on the Gay Pride movement. "I don't see autism as a disability," says Nelson. "I see it as another human variation."

Today, an annual event called Autscape is happening in Somerset. It is a three-day retreat run by and for people on the autistic spectrum, where behaviours seen as odd by neurotypicals (the name that those on the autism spectrum have given to those who aren't) - such as stimming (repetitive movements) and impaired social skills - are considered normal. The environment is kept as autism-friendly as possible, with quiet spaces for people to go to and fluorescent lighting, which can be a source of pain or irritation to autistics, kept to a minimum.

More than 500,000 people in the UK are on the autistic spectrum and four times as many boys are affected as girls. People with Asperger's syndrome share some of the features - such as narrow interests and poor social skills - that many with classic autism have, but without their learning difficulties. Nobody knows what causes it and a "cure" seems to be unrealistic, although a huge amount of money is ploughed into researching just that. Nelson and AFF have a different aim. They want to get autism recognised as a minority rather than a disability, believing that existing disability discrimination laws don't protect those who are not disabled but who "still have something that makes them look or act differently from other people".

"The classic example," Nelson says in his slow, careful sentences, "is someone going into a job interview and looking a bit odd, maybe not making eye contact, but otherwise having excellent qualifications. They probably won't get the job."

But Nelson is something of a radical, and the autistic community is split on his view. Larry Arnold, the first autistic person to join the board of the National Autistic Society (NAS), thinks the AFF "are politically naive". "I believe in universal anti-discrimination laws," he says. "I don't agree with this separatism."

Arnold does, however, subscribe to the idea that autism is part of neurodiversity and believes that many of the difficulties autistic people face are due to society rather than the condition itself. "It's all part of the societal construction of disability, it's seen in negative terms and is portrayed in negative, pitying advertising by organisations raising funds for research which wants to eliminate us from the planet. That's a very American style of campaigning and I'm seeing it coming over here. I would like us to have a greater say in organisations that purport to speak on our behalf. People say 'it's all right for you, you can talk and you were able to get a university degree, whereas our children can't do this, that or the other'. But I'm for valuing every level on the autistic spectrum, it doesn't matter how able or unable you are to carry out 'normal' functions that are dictated by society. I think there is something of an autistic culture developing in this country and worldwide."

Indeed, among neurotypicals, autism has become a fascination. The success of books such as The Curious Incident of the Dog in the Night-Time, by Mark Haddon, Daniel Isn't Talking, by Marti Leimbach, who based the book on her experience of bringing up her autistic son, and Send in the Idiots, by Kamran Nazeer, has fuelled what one publisher called "spectrum publishing". Second Life, the online virtual reality world, has a place where autistic people go to meet (geographical distances aside, many of the users would find it impossible to meet face-to-face because of their lack of social interaction skills). It is also home to the Autistic Liberation Front; you can buy T-shirts and badges affiliated to it with slogans such as "I am not a puzzle, I am a person" and "not being able to speak is not the same as not having anything to say".

On YouTube, the online video posting site, Amanda Baggs, a 26-year-old woman with autism who lives in the US, became one of the most-watched stars of this year. Her eight-minute film, In My Language, provided an insight into her life and showed an ability to communicate, via computer and voice synthesiser, that shatters the view that autistics are "locked in a world of their own". Although she finds it impossible to communicate verbally, she is able to type very quickly and her blog shows that she is articulate and funny.

Right now, Baggs is the most visible spokesperson for the autism self-advocacy movement in the US, a movement that is all the stronger for the aggressive fundraising there for a cure for autism. Five months ago, the Combating Autism Act was passed by Congress, with the government authorising $1bn to be spent on research, including genetic screening. Among those trying to take a different approach is Valerie Paradiz, who has Asperger's syndrome. She founded the School for Autistic Strength, Purpose and Independence in Education (Aspie) in New York State when her son Elijah, who also has AS, was a child.

At Aspie, autistic children are taught that having autism is a strength rather than a disability. "In most schools, kids with Asperger's syndrome are placed in settings that are either overwhelming in a social or sensory way, or underwhelming in an intellectual way and many adolescents end up struggling with profound isolation and depression," she says. "I actually see educational access issues for these kids as something very similar to wheelchair access." Autism, she says, is "not a pathological condition or a disease, but a way of life that possesses a culture and history all its own". Although the school closed last year, another will be opening later this year.

Jim Sinclair was one of the first activists in the self-advocacy movement. An autistic person who didn't speak until he was 12, he lives in New York state. "There were plenty of people talking about 'autism rights' before me," he says. "It's just that most of them were talking in terms of advocacy for rather than by autistic people. They were mostly parents, speaking up about what they thought were important issues and they did a lot of good things, in terms of advocating for more community services and opposing things like the use of [harsh behavioural] 'therapies'. But I think I was one of the first autistic people to promote the idea that autistic people can and should speak up for ourselves."

Sinclair's 1993 essay, Don't Mourn for Us, which was published in the newsletter of Autism Network International, and given as a speech to a conference on autism, is a foundation of the movement and was addressed to parents who are devastated by the diagnosis of an autistic child. "I still get emails from people telling me that [the essay] has completely changed the way they view their children or themselves."

Back in the UK, in Cumbria, Mike Stanton is a special needs teacher whose son, Matthew, now 22 and at university, was diagnosed with Asperger's syndrome when he was 12. Stanton believes that the big problem for parents whose children have just been diagnosed as autistic is that they are told to do everything they can to "rescue" their child from autism. "Whether that's going to America to have them chelated [a therapy that removes metal toxins after it was suggested that autism could be linked to mercury poisoning] or changing their diet or any number of other things.

"Autism is seen as this devastating condition but accepting the child and working on the child's strengths will help them to develop positive self-esteem," he says. "We wouldn't get Nobel prize-winners with autism if there weren't some strengths to autism." The many who are considered, in hindsight, to have been somewhere on the autistic spectrum include Albert Einstein, Isaac Newton and Marie Curie. Many Aspies believe that Bill Gates shares some of their traits.

But how do some parents of children considered to be low-functioning feel about a cure? Megan is seven and at the low-functioning end of the spectrum. Her father, Kevin Leitch, writes a blog about his experience of bringing her up. "When she was diagnosed, it was shocking beyond belief," he says. "We felt like our world had been taken away. But in a reasonably short space of time, we realised Megan wasn't dying, she wasn't ill. I don't want to paint a rose-coloured view of our lives, but it's far from doom and gloom. Megan still has meltdowns, or tantrums, because she finds it very difficult to communicate. She can self-injure and at other times go for us. But there are things that Megan brings to our lives that more than compensate for the difficulties."

If a cure were found tomorrow, he wouldn't give it to his daughter. "I want to put her in a position where she can advocate for herself what she wants," he says. "She has speech therapy, because we want to give her every opportunity to make her life easier, but my personal view is that it's not necessary for an autistic person to be 'cured'."

His view is shared by those who are involved in researching autism. "I do think there is a benefit in trying to help people with autism-spectrum conditions with areas of difficulty such as emotion recognition," says Professor Simon Baron-Cohen, the director of the Autism Research Centre in Cambridge. "Nobody would dispute the place for interventions that alleviate areas of difficulty, while leaving the areas of strength untouched. But to talk about a 'cure for autism' is a sledge-hammer approach and the fear would be that in the process of alleviating the areas of difficulty, the qualities that are special - such as the remarkable attention to detail, and the ability to concentrate for long periods on a small topic in depth - would be lost. Autism is both a disability and a difference. We need to find ways of alleviating the disability while respecting and valuing the difference."

An estimated 80% of foetuses with Down's syndrome are aborted and autism rights campaigners are worried that the same could happen if prenatal tests for autism are developed. "Prenatal tests may well materialise," says Baron-Cohen. "For example, we are looking at foetal testosterone levels during pregnancy, and many groups, including ours, are looking at genetics. But these lines of research need not be towards a cure - rather, they may lead to more reliable as well as earlier diagnosis, which can mean that interventions and support for the child and their family can start sooner." But others, especially Aspies for Freedom, fear that the development of a prenatal test is the thing that will wipe them out.

"I don't want to get to be an old man," says Gareth Nelson, "and know that there will be no more people like me being born".

· Gareth Nelson's site is Aspiesforfreedom.com