There's a moment in The King's Speech, the new film about King George VI and his stutter, when the king (played by Colin Firth) meets his new speech therapist for the first time. The therapist is Australian-born Lionel Logue, played by Geoffrey Rush, and he informs HRH that "we need to relax your jaw muscles". Firth swallows nervously, the tendons in his neck standing out, jaw muscles far from relaxed. He looks terrified, and eventually barks out one word: "Fine."

It is a startling performance from Firth, though not many would know it. He has captured it perfectly: the fear, the dry, panicked swallow, the unendurable tension, the feeling that your jaw and/or throat is just about to seize up . . . I know this because I've been there; welcome to the world of the stutterer.

According to the online Dictionary of Australian Biography, Logue practised at 146 Harley Street, London, from 1924 and "fees paid by his wealthy clients enabled him to accept poorer patients without charge". Sadly, Logue died in 1953 and, anyway, would probably not have made the trip to east London, 10 years later, when a boy of five started to stammer. Where it came from I have no idea, but it seemed to coincide with the removal of all my teeth by a butcher in the NHS dental department at Whitechapel hospital, and the start of primary school. Of course, "teeth" and "Keith" rhyme, so you can imagine the taunts.

When the time came to leave primary school and move on to grammar school I knew I had to solve the problem or go down fighting. Which is basically what had happened at the primary school, but punching people in the head was never going to be a long-term solution – despite what my dad said.

My grades were pretty good but I refused to speak in class unless singled out. To this day I suspect there were teachers who never knew there was a panic-stricken boy at the back for whom every class was a heart-thumping hour of terror.

My parents were no help: they were working-class to the core and had neither the inclination nor the money to help. Logue would have been out of our league, and as for seeing a shrink – well, that meant you were mad, right?

And while there is a little more understanding of the problem today, it's still what British Stammering Association (BSA) chief executive Norbert Lieckfeldt calls "the hidden disability", which is why the BSA is calling The King's Speech a "once-in-a-generation moment to create change and to increase awareness" and organising a national campaign around it. Stammering is one of the most common speech disorders, affecting something like one in 20 children (and is three to four times more common in boys than girls).

Lieckfeldt, who has seen the movie three times already, says it portrays stammering in a way that is "not a pastiche, not something funny, not a dramatic device to inject humour . . . Colin Firth portrays a person who struggles with stammering, but it's not funny and he's not made out to be mad."

"The thing is," laughs Lieckfeldt, himself a stammerer, "we are quiet, we're not very good at talking about it because, well, we have difficulties talking about it. Very often we talk to teachers who say: 'I've been teaching for 20 years and never had a stammering child' and we say: 'That's just statistically not possible. You've had them but they pull the wool over your eyes.' And that's what people do – they hide it."

This is echoed by Elaine Kelman, a speech and language therapist at the Michael Palin Centre for Stammering Children in London (Palin's father was a sufferer and Palin mined his experience for the role of stammering Ken in the film A Fish Called Wanda). Kelman told the Guardian last year: "These children are experts at hiding their stammers. They may not put up their hand or ask a question, and some will clown around."

Oh, Ms Kelman, where were you in 1971? I still wonder why I a) tried out for First Witch in our class production of Macbeth and b) decided to study French and German for A-level. Imagine the scene. There is thunder and lightning and the First Witch says:

"W-w-w-w- when shall we three m-m-m-m-meet again,

In thunder, lightning, or in rain?"

It ruins the metre.

French and German O-levels were no problem – there was no real oral component – but A-levels were a different kettle of fish (no doubt called W-W-Wanda). The orals were held off-campus. To this day, I have no idea exactly where it was. Thinking back to that day is like having synaesthesia – I try to recall my French and German orals and all I get is a fizzing red mist through which the vague faces of those poor examiners stare, trying to make some sense of the gargling, stammering, teeth-grinding, red-faced fool standing in front of them and, seemingly, trying to swallow first his tongue and, if possible, his whole head.

After that I didn't want to go on to university and have it start all over again. I didn't want to meet new people who would laugh at me. I didn't want to work extra hard because my throat and jaw and brain wouldn't play ball. So I stayed in the shop I had worked in part-time since the age of 13. They knew me, they knew the stammer; I could relax.

But the pull of newspapers was too strong. At 20, I applied to my local newspaper. By this time I had somehow managed to wrestle my stammer under control to the extent that, unless tired or stressed, I only had problems with a few words.

I still don't know how it works; I can say "welcome" or "wormhole" or "winsome" but, for some reason, get totally stuck on "women". Same for "seven"; "sentient", "sex", "sell" – all fine, but try getting me to say "seven".

It was then, I think, that I began to word-switch. Thinking ahead, visualising my sentences written on a page before they came out of my mouth. I would know that a difficult word was coming up and would switch it to one I could say. It did wonders for the vocabulary but numbers harbour problems. Imagine my despair at discovering my new workplace phone number: 729-1414. There it was: the dreaded seven. How many other people remember the phone number of the office they worked in 32 years ago?

Every phone call became a nightmare. They are going to have to get back to you; what's your number? Well, if you must know it's "ssssssssssssssssssssss-ssssssssssssssssseventwonineonefouronefour". Once I got started, the only way I could finish was by running out of breath – the rest of the number came out like joined-up handwriting and I'd finish up gasping for air like a drowning man.

"What?" they'd say, and I'd feel like crying.

One day, after a particularly sibilant episode on the phone, a colleague laughed at me. I turned on her: "What? What are you laughing at? You think this is fucking funny for me?"

I tried explaining my predicament to callers: "My number? Yes, of course. Now this might seem a little odd but, you see, I have a stutter and one of my problem words is the first word of my phone number. And that first number is the number after six."

"Sure," came the reply, "I'll ring you back after six. What's your phone number?"

"Oh, look it up," I growled, and slammed the phone down.

Today it's no longer an issue. Well, not for other people. A friend professed surprise that I even had a stammer. I would like to say I have beaten it, but it's not true. I have beaten it into submission but, like an alcoholic, the beast is always there, lurking in the background.

I did it by word-switching but also by changing the cadences of my speech, which now include a multitude of hesitations, verbal ticks and allied hand movements, like a magician's legerdemain. Throw all this at someone and they tend to miss the main trick.

It works, but not always. My partner says she can hear it when I am tired or stressed. I still have problems with "seven" and have to focus my mind and concentrate very hard on it when I "see" it coming in a sentence.

Fortunately, there is more help available for stutterers than ever before. Lieckfeldt says early speech therapy is often the answer: "Children start to stammer, usually, at around two-and-half to three years old, when they start to form complete sentences. Between 5% and 8% of them then go through a phase when they become less fluent, but the vast majority of them will recover normally.

"What we say to parents is that if you are worried, go to see a speech language therapist as soon as possible. If you can catch it at that age, the chances of a complete recovery are very, very high but if you wait until six or seven, the success rate in terms of total return to fluency drops like a stone. From seven or eight onwards, total recovery is very rare."

And he adds: "We want to get people to speak out and let people know that it takes courage to live with a stammer. If you have a stammer and you are a 15-year-old who gets bullied at school and who gets laughed at at school and yet goes to school every day – that takes guts."

Last year, the then Department for Children, Schools and Families commissioned the Palin centre to create the Stammering Information Programme, which includes a DVD presented by young children who stutter. The messages the children, aged two to 18, wanted to get across included not being told to hurry up and finish a sentence, their feelings of sadness, embarrassment and fear, of being thought less intelligent, of being ignored and overlooked in school. Samuel Zack, aged 10 when the DVD was made, might be describing my schooldays when he says: "I can't do presentations. I can't do things like talking in front of people and it makes me not answer a lot of questions when I know lots of them."

Sadly, due to all the s-s-s-stuttering, the DVD is s-s-s-seven hours long. Come on, you've got to laugh or you'd cry.

For more information visit stammering.org or call 0845 603 2001. There is also a Facebook page.