It was Susan Hampshire’s worst nightmare come true, and every actor’s dread, when for the first time in her long career she “dried” on stage.

“It was as though my memory had run away on a ticker tape,” she recalls of the recent incident in Black Chiffon on tour at the Theatre Royal, Windsor, in which she plays the leading role.

“I knew I was on stage and I knew I was playing a character called Mrs Christie who was talking to a Dr Hawkins - but that was all. It was the last scene in the play and I just said: ‘I’m sorry doctor, I’m so upset you must excuse me,’ and walked into the wings. The prompter told me what I had to say next and I went back on, progressed the story a bit more, ad-libbing as I went, and then had to excuse myself again because I had run out of words.”

For Susan Hampshire a glance at the prompt script, which would have helped most actors in her predicament, was useless. Being dyslexic she couldn’t read the closely-written type. “Forgetting my lines was nothing to do with my dyslexia,” she insists. “But being dyslexic made it difficult for me to cope.”

The curtain was brought down and it was explained to a stunned audience that Miss Hampshire was unwell. Someone was sent off in search of the star’s spectacles and her special personalised copy of the script with its multi-coloured hieroglyphics and highlighting and all ended well. “As soon as I’d read about six lines of my own script I was fine. I went back on and finished the play.”

The audience enjoyed witnessing this rare glimpse of the perils of live theatre, but for Susan Hampshire the rest of the evening passed in a blur. She had trouble finding her way as she drove home and, feeling disorientated, she couldn’t remember the sequence of her home routine at all. After a good night’s sleep, however, she was fine.

Like all dyslexics, she has good and bad days. On that eventful evening she had gone on stage after rehearsing all afternoon and not having eaten. “My dyslexia always seems worse when I am tired or upset,” she says. “That day I suppose my brain was just overloaded. I find it incredibly hard to learn my lines, but having done so my memory is really very good.”

Susan Hampshire, now aged 53, has single-handedly probably done more to bring dyslexia out into the open than anyone in the public eye. She was not diagnosed as dyslexic herself until she was 30 and by chance met a member of the Kershaw Committee which pioneered research into dyslexia in adults. By then she was already an established actress who had endeared herself to the nation with her memorable performance as Fleur in the television production of The Forsyte Saga.

Admitting her problem publicly then, when dyslexia was scarcely recognised medically, let alone educationally, was “incredibly brave” says her agent, Ros Chatto, who at first tried to dissuade her but now says, “I don’t think Susan has ever lost a part through being dyslexic. It is never a problem from my point of view as her agent. It’s a problem for her, because she has to work so hard to learn her lines. I think dyslexia has actually made her more determined.” As evidence she cites the actress “learning French in a week” for a film she made with Charles Aznavour called Paris in August.

“I think dyslexics have a natural aptitude for languages,” explains Hampshire. The director of the film, Pierre Granier-Deferre, became her first husband and she is now bi-lingual. But she is also bi-dyslexic, having equal difficulty reading French as English. Her son by Granier-Deferre, Christopher, is dyslexic too.

“I noticed when he was about nine that he kept misspelling his thank you letters at Christmas, but when I asked his teachers if they thought he might be dyslexic I was told he was simply confused because he was speaking two languages,” she says. Unconvinced, she took her son to the Dyslexia Institute for an independent assessment which proved that, as she suspected, he was dyslexic, though less severely than his mother.

After the official diagnosis, Christopher was allowed an extra 20 minutes in exams which, says his mother, changed his life. “He had time to read the questions properly and to read through his work afterwards. Dyslexic people often reverse things, make something negative instead of positive, misread questions and therefore answer them wrongly. It might say, for instance, ‘answer one of three’ and you read it as ‘answer all three’.”

Having been warned at school that he was unlikely to get into university, Christopher gained two As and a B at A-level, went to Goldsmiths’ College at the University of London and is now a first assistant film director with Merchant Ivory in Paris.

Susan Hampshire was less fortunate than her son in her early battles with dyslexia which was unrecognised when she was a child. Her mother thought she was retarded. She was the youngest by several years of four children. Her brother and two sisters were very academic and her father had graduated from Oxford with a double first in science. Susan, however, couldn’t even read her letters by the time her peers were beginning to read books. She confused “b” for “d” and “f ” for “t” and struggled as much with short words such as “it”, “so” and “of” as she did with “because” and “therefore”.

Her resourceful mother set up a school, Hampshires, which still exists in London’s Knightsbridge and where one of her sisters still teaches, which Susan attended until the age of 16. By nine she had mastered writing her own name (until then she always referred to herself as S H) and by the age of 12 she was able to read, with difficulty.

She is quick to praise the help and support she received from patient friends and family. “Everyone was very kind to me. I was never teased because I couldn’t read. Dyslexia drove me to tears, but they were tears of frustration, not because anyone tormented me.” She remembers one particular friend, Patricia Beckwith, reading the classics to her for four hours at a stretch. Her sisters, Jane and Ann, bought her picture books much younger than her years to encourage her to read. One memorable day she wrote to Winston Churchill, who was then Prime Minister - but it took 35 attempts before it was good enough to be posted.

Today, 40 years after her debut in Emlyn Williams’ Night Must Fall, at Bognor Regis, Susan Hampshire still finds learning lines a long and tortuous business. “The first reading of a new script is just a nightmare. I can’t skim because I am always worried that I might miss the bit which is the real reason for accepting the part.” She reads every script at least three times before deciding to take it on and each read-through takes her five hours, usually in several instalments. She finds early morning the best time and gets up at 5.30 to begin, usually waiting until the evening before tackling the next section. If an early answer is needed, she takes the telephone off the hook and settles down to read in the afternoon too. Sometimes she works through the night.

Reading scripts, she freely admits, is the worst part of her job. “It takes so long because I don’t know the story. If I know what a play is about it is much easier to read it. Finding the five hours to read isn’t difficult, the hard part is finding the courage to begin. All that close type is what I find so daunting.” Television scripts she finds much easier because they are written in short sentences with lots of space on the page.

Contracts are a minefield too. She has found herself agreeing to appear in plays for 16 weeks she misread as running for six. Sometimes she has misread fees and has ended up both worse off and better off than she expected.

Having accepted a part she has the script enlarged to A4 and three copies made. One is “for show” which she carries around and, apart from a few pages from a sticker pad with notes to remind her of important moves or the emotions she should be feeling at a particular juncture, has little marking on it. Another copy has her lines marked with a highlighting pen. This one she keeps beside the bed so she can revise late into the night.

The “master script” is a riot of bright colours and hieroglyphics. One colour marks her own part, another her cues. Important words in other actors’ lines that refer to her character are marked in a third colour and a fourth is used for the dots she puts to show when to take a breath. When a particular emotion is required the script is marked in a fifth colour and every page is scattered with yellow stick-on pages with additional notes in pencil or red ballpoint. She has taken years to develop this rainbow technique, but once she has amended her script she says she finds it very easy to read.

“Funnily enough I never have trouble reading my own writing,” she says. “All the books I have written have been in longhand on a yellow legal pad which is much easier for me to decipher than words on a white background.”

Learning her parts is also a matter of well-practised routine. First, she learns the story thoroughly, then tries to commit five pages a day to memory, repeating the words over and over again out loud. Often she studies locked in the sanctuary of her own bathroom because she knows that there no-one will interrupt. “My scripts get damp, wet, curled up, because sometimes I learn them lying in the bath. Sometimes I might be sitting on the lid of the lavatory seat.”

Some parts demand so much of her she has to leave home. She checks into a small hotel for a few days, never leaving her room. “I read, have a cup of tea, read, have a cup of tea . . . There was a tremendous amount of extra reading for The Pallisers, which I found difficult. I hadn’t read Galsworthy before I did The Forsyte Saga but luckily I found his work quite easy to read.”

But words are not Susan Hampshire’s only problem as an actress. Like many dyslexics, she cannot tell left from right either. Pretty quickly she learns to associate stage directions with moving towards a chair or some other piece of furniture, fervently hoping that nobody will re-arrange the props.

Shooting the film Living Free, based on Joy Adamson’s book about lions, proved particularly hazardous because of this. “I found myself chasing a semi-wild lioness around the bush in Kenya with the camera crew 100 yards away, unable to come to my rescue if things went wrong. I was told to run right and veered left, running straight into the lioness. If she hadn’t had a hat in her mouth that mistake would have cost me my life. From then on I wrote R on my right palm and L on my left, something I still do when it is really essential that I remember which way to move.”

For an actress with dyslexia reading from a teleprompter is no easier than reading from a normal script, and Hampshire still remembers with horror reading a children’s story on the television programme Jackanory. “I tried to learn the whole book, sitting up night after night, because I had only four days until the recording. Once inside the studio I looked at the teleprompter but I couldn’t read it. The more I panicked the less I could see. I pretended to read - but everything was a blur - and when the camera focussed on my face, my blank expression was obvious.”

Another experience she prefers to forget was being asked to dub her own voice in the French versions of The Forsyte Saga and Paris in August, reading from a script into a microphone. “I went into the recording studios to do both but they were very long parts and after about half a day I just had to throw in the towel.”

Over the years Susan Hampshire’s dyslexia hasn’t improved, but she has learned to come to terms with it. “She is a role model for so many people,” says Liz Brooks, executive director of the Dyslexia Institute. “By being prepared to stand up and say she has these difficulties she has inspired others.”

Susan Hampshire is now president of the Dyslexia Institute and a very hard-working one. Recently she single-handedly raised Pounds 40,000 for a bursary to provide special lessons for children. She has honorary doctorates from the City University, London, St Andrews, Scotland, Kingston, Surrey, and Boston, USA, for her work for dyslexics and last year she was awarded the OBE.

Other actors - including Zo Wanamaker and Anthony Hopkins - have admitted being dyslexic, but Susan Hampshire is still the person whose name is instantly associated with dyslexia in the public mind. It is a situation which has, co-incidentally, added to her problems. Her diaries (she has three of these, too) are crammed with multi-coloured markings and stickers pinpointing the meetings and charity appearances scattered among professional engagements. She receives at least 40 letters a week, mostly concerning dyslexia, all of which have to be carefully, but laboriously, read and answered. “It’s nice that people write to me, but it takes two whole days of my week to reply to them.”

Her husband, Sir Eddie Rulukundis, the impresario, cannot understand why she writes when in some cases a phone call would be sufficient. Susan Hampshire explains that often she misdials phone numbers or finds she’s written them down wrong. Anyway, she concludes, “I was brought up to write letters.”

* The provincial tour of Black Chiffon ends in Newcastle on April 20.

The Dyslexia Institute has produced three new videos aimed at teachers, parents and people with dyslexia. They will be shown in the Learning Zone on BBC 2 at 5am every Wednesday for three months from April 3. The videos cost between Pounds 10 and Pounds 15 and are available from the Dyslexia Institute, 133 Gresham Road, Staines, Middlesex TW18 2AJ (01784 463851) Notable Dyslexics

Einstein, Leonardo da Vinci and Rodin are all thought to have been dyslexic.

Deputy Prime Minister Michael Heseltine, architect Richard Rogers, swimmer Duncan Goodhew, singer Cher and racing driver Jackie Stewart are among those in the public eye who have admitted their dyslexia.