In a quiet Bristol suburb, Helen Johnson holds a long lead at the end of which is Percy, a large black labrador. Alongside walks her six-year-old son William. A normal family walk, you might think. And it is. For the Johnsons, however, such a walk, even just around the block, is a new and wonderful achievement. And it is all due to three-year-old Percy, the UK's first autism assistance dog.

William suffers from autism and learning difficulties. One in 100 children is believed to be on the autistic spectrum, although only some will be as severely affected as William, who rarely makes eye contact, is still in nappies, and has no recognisable language. He becomes distressed by changes in routine and he hates walking - or used to. Without Percy, says his mother, he'd sit in the middle of the pavement within minutes of setting off and refuse to move. He might grizzle, cry or flap his arms in classic autistic repetitive behaviour. “We had reached the point where we avoided going out. It was just too exhausting.”

Hardly surprising then that six months ago the Johnsons leapt at the chance to become the first family in Britain to receive an autism assistance dog as part of a two-year pilot project by Dogs for the Disabled. Dogs have been used to help autistic children in North America for several years and, since 2005, in Ireland. Now, 16 families in the UK, with autistic children aged between 3 and 10, will receive dogs before the end of next year. If they prove beneficial - and if funding allows - more will follow.

Dogs for the Disabled has long trained dogs to help physically disabled people, but it began to partner them with children in 2004. Helen McCain, the charity's director of training and development, admits that training dogs to help autistic kids is quite a departure. Rather than teaching the dog to carry out specific tasks, such as emptying a washing machine or opening doors, it is a matter of selecting a dog with a calm temperament and teaching it to follow instructions, and to stay steady and affectionate whatever is going on around it.

After more than a year of socialisation, followed by about nine months of formal training - and a bill of £10,000 - Percy is “Mr Chilled”, says Helen Johnson. “Nothing fazes him.” This is more than can be said of even the most laid-back parents, particularly when their child throws a wobbly in the middle of a shopping centre. “I get stressed and uptight, which only makes matters worse,” she admits. “Percy doesn't and, as a result, it actually helps to calm the situation down.” Helen McCain agrees: “Dogs don't come with emotional baggage and they don't have to think about the shopping. They aren't worried about what a child might do. They are non-judgmental and unconditional.”

There is something about being with Percy, Johnson says, that calms William and makes him more focused. “We don't really know why it works,” she says, “but it does.” Evidence, albeit mainly anecdotal, has been building for some time to support the idea that dogs (and occasionally horses) can help autistic children to connect better with the world around them.

William has no sense of danger

Irish Guide Dogs for the Blind (IGDB) carried out a pilot scheme with eight children in 2005 and found that they all became more willing to visit new places and more aware of danger. Some families also reported fewer tantrums and a marked improvement in behaviour, independence and language development. Two American studies reported similar findings.

As we leave the Johnsons' neat, modern home to go for our walk, William immediately wanders towards the kerb. Even though his mother calls him back repeatedly, explaining that he has no sense of danger, he doesn't respond. Percy, meanwhile, bounces around like any excited pet dog. Until, that is, Johnson produces “the jacket” and “the belt”. She places these on the dog and the child and attaches them together. The atmosphere is instantly calmer.

William doesn't bolt, as do many autistic children, but he does wander, and Percy is trained to sit when he feels a sustained tug on his lead, “anchoring” his young master. Once Percy's jacket is on he knows he is “working” and, usefully, so does everyone else. It makes it obvious that William isn't just naughty. Before Percy joined the family, Johnson would get dirty looks from passers-by if William started playing up. “We still draw attention,” says Johnson, “but it is all positive. Percy does our explaining for us. I no longer have to justify my son's behaviour.”

Not only is William tethered to Percy, but he also holds a handle on the dog's jacket. Johnson directst Percy with a string of gentle commands and encouragement, and William walks happily beside him. Johnson has no need to nag her son. She doesn't even have to ask him to stop before we cross roads. She directs Percy, and William follows, smiling, babbling and humming to himself.

“This is a lovely project,” says Richard Mills, the director of research at the National Autistic Society. Unlike many “treatments” for autism (mainstream drugs and alternative therapies), it involves no risk. “It is hard to see any downsides and it helps the whole family,” he says. Mills is working with Dogs for the Disabled on a protocol to record the results of the pilot as scientifically as possible. The aim is to begin to understand more about why and how the dogs help and who is most likely to benefit.

Meanwhile, Johnson is delighted simply to be able to walk with her son. We are on the way back to the house when William suddenly trips and falls on the pavement. Johnson picks him up, cuddles and reassures him, then offers his hurt hand to Percy to lick. William strokes Percy's head and stops crying. He puts his free hand in mine and the four of us walk the rest of the way home.

A happy tail

Nuala Gardner and her son Dale, now 19, have first-hand experience of the impact a dog can have on an autistic child. Dale has severe autism and, as a young child, didn't speak, show affection or understand emotions.

When he was 5, the family visited a cousin who owned two Scottie dogs and, to everyone's surprise, Dale spent the day happily playing with the pair. The Gardners decided to buy a golden retriever puppy, Henry, and, within weeks, the teachers were saying Dale was happier and related to other children better, and was improving faster in all areas.

“I found faces scary,” says Dale, now at college studying for an HNC in childcare. “I would misread expressions. But Henry had such a calm and friendly face and he never looked angry. I could look at him and it took away the stress of talking to people.”

To find out more about the pilot scheme, call 01295 252600 or visit www.dogsforthedisabled.org

Philip Millard is running the London Marathon on April 13 to raise money for Treehouse, a charity for the education of autistic children. Go to themillards.groups.timeshealth.co.uk

Pets and health

Blood pressure A study in Australia of 5,741 people showed that those with pets had significantly lower blood pressure and blood- fat levels than non-pet owners.

Anxiety Researchers at the Virginia Commonwealth University in the US took animals into a psychiatric hospital to visit 230 patients. They found that interaction with an animal significantly reduced patients' anxiety.

Depression In a study last year, Italian researchers found that dogs lowered depression among patients undergoing chemotherapy.

Foster friendships A University of Warwick study in 2000 showed that being accompanied by a dog meant more interactions with strangers.

Exercise A 2006 US study of 1,282 dog-walkers concluded that owning a dog encouraged people to take more exercise.

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