In the spring of 1985, I went to an unusual tea party at the House of Commons. Four children's charities had come together to raise money, and I had been invited there to celebrate the culmination of the campaign and to collect a cheque on behalf of one of them.

Each had been allowed to bring along a group of children and, when I arrived, the room was already noisy with excited laughter and chatter.

But one table was different. At first, I couldn't think quite why it stood out from the other three - the children had no obvious disability and looked quite "normal". Then, after a few seconds of watching, I noticed that not only were the children unusually quiet, but that there was no interaction between them; no shouting or joking, not even any arguing.

They didn't look at one another - indeed, they seemed completely unaware of each other's presence. The difference between their behaviour and that of the other children was so marked that I asked one of their helpers where they were from.

I had heard the word "autism" before, but knew almost nothing about the condition. My attitude to it and knowledge of it, I suspect, was fairly typical of the general public in those days: autism was a condition similar to dyslexia, perhaps - a problem in communication, an emotional "shutting in". I certainly had no idea of its real implications.

As I listened to the helper's explanation and went on watching the children, I began to understand just how devastating and drastic were the effects of the simple sounding "social impairment" of autism.

She described the experience of autism as similar to being in a private world. But it was seeing the children at that table utterly failing to communicate with each other that really brought home to me just how terrifying and lonely it must be to look out on a world where everything is mysterious and incomprehensible. When their teacher asked if I might be able to come and open a holiday playgroup the following month, I jumped at the chance to find out more.

The time I spent with the children on the morning of the opening made me even more determined to do whatever I could to help to promote awareness of autism. The experience made me realise just how much we take for granted when we communicate with each other and the world at large: to those children, everyday social signals were meaningless and confusing. Why did I smile when I greeted them? What did I mean when I said "hello" to them? And - above all - what was I threatening as I stretched out my arms towards them?

I visited several more schools over the next few years, and gradually learnt more about the sensitivities and problems of those with autism. For example, the nuances, exaggerations and pretences of "normal" conversation can be taken literally: I soon learnt not to use expressions such as "it's raining cats and dogs" or "I could eat a horse".

One little boy I got to know was terrified of going for a walk: it took some time for me to realise that the simple assumptions I make every time I leave the house just don't apply in his world. He was convinced the pavement wasn't solid and that he would fall through it.

And a 12-year-old girl proved impossible to get through to until her hearing problems were diagnosed. It transpired that every tiny sound was intolerable to her: even the noise of the person next to her breathing was unbearable - a hissing, sawing rasp that threatened unmentionable horror.

Grasping the complexities of "self" and "others", I found, can be impossible for a person with autism. I remember the confusion I felt the first time a small boy grabbed my hand and moved it towards a glass of orange. Later, I learnt that this is a very typical way for someone with autism to ask for a drink - as if a need can be expressed only by assuming the third person.

In trying to learn how to speak, many people with autism will use "echolalia" - the uncanny, parrot-like repetition of phrases that are said to them. A little boy at one of the schools I visit has learnt my name almost too well, and now uses it, in full, several times during the course of every sentence. His greeting to me tends to be "Hello, Jane Asher, I'm very well, Jane Asher, what time is it, Jane Asher, I want a drink, Jane Asher", and so on.

Routine is crucial: the need for it that I found in my own children when very young is exaggerated and persistent in someone with autism.

This was brought home to me last month, when I went to see a new autism resource base in a special school; the parents of the children currently being taught there were invited to meet me. On seeing his Mum, one of the boys immediately went to fetch his coat and insisted on putting it on and making his way out of the building. It took a lot of encouragement to make him understand that school was not over and that he should stay.

Another child will tell me not only the make, model and colour of the hire car that I have arrived in, but those of each car from all my previous visits. The idea of the autistic savant is generally misguided, but there is no doubt that many people with autism show extraordinary ability, particularly in areas where repetition, memory and logic are involved.

This passion for repetition can mean that many adults, if found the right jobs, can work efficiently and with satisfaction. But it can sometimes be hard to predict every potential problem. I was delighted when a young man whom I had known at one of our schools was found a place working in a supermarket. He would efficiently and neatly stack the shelves, but no one had foreseen just how upset he would get once the shop was open and customers insisted on taking the tins away from his display.

There is no doubt that the study of autism is fascinating: all the more so because many aspects of it are oddly familiar. Working with people with autism over 13 years has made me consider the strange business of just what is "self", and just how much we all live behind a wall of pretence and supposition - whether we call it social interaction, manners or civilised behaviour.

An intriguing study a few years ago suggested that people suffering from clinical depression are more in touch with that great abstraction, "reality", than the rest of us who are supposedly sane; in some ways I feel the same about autism.

As I go about my daily business, pretending that the world really makes sense, I can easily imagine that on the flip of a coin, it could all be revealed as utterly incomprehensible and terrifying. Fear in a handful of dust, indeed.

For me, the most heartbreaking aspect of autism in children is the lack of emotional response. A large part of the happiness my husband and I derive from our three children is in the spontaneous and unthinking expressions of love that pass between us every day: a quick kiss as they leave for school, a cuddle when they're upset or a whispered "I love you, Mum and Dad" at bedtime.

It's easy to take these simple things for granted. For the parent of a child with autism, such everyday joys may not exist, and the natural instinct to reach out and hug a frightened, shouting child may not be enough - or may well be rejected and misunderstood.

A six-year-old boy I first met at that playgroup 13 years ago used to sit alone in a corner, sobbing uncontrollably; any attempts by me or any of the staff at that stage to get through to him only made him even more frightened. I used to find that almost unbearable.

But now, after years of patient, professional help, he has become a contented, calm young man who will allow me to take his hand, and will even - if I am lucky - suddenly rest his head on my shoulder.

One thing is certain: if the right help is given, early enough, enormous gains can be made and the potential of the individual can be realised. Children are still being diagnosed far too late, and many of them fail to find the care close to home that their families so desperately want for them.

And it isn't only children that need help. I have found it hard enough to get support and sympathy for the plight of attractive, appealing young children with autism; when they grow into difficult, uncooperative adults, it can be impossible.

A young man in one of our units, who was not diagnosed until adulthood, regularly screams at night, smears his faeces on the walls and attempts to mutilate himself: the horrors that torment him are unthinkable.

Recently, I helped to launch a new residential home for seven adults with what is now called "severely challenging behaviour". The day of the launch was joyous: we celebrated the triumph of hard work and fundraising that had achieved such a vital and humane provision.

But the seven people on whose behalf all this work had been done were missing - taken out for the day to avoid the noise and excitement that would have proved far too disturbing. If we had only been able to reach them as children, those seven guests of honour might well have been there to enjoy the inauguration of their new home, without fear and distress.

From the December 25, 1998 edition of the Daily Telegraph