I'm sitting in a hallowed West End theatre, happy to have scored a ticket - not a cheap one, either - to a sold-out American play. The show starts, and suddenly my ears prick with discomfort. What are Cornish accents doing in 1930s Michigan? What American native has ever said “warter” instead of “wadder”?

Yes, it's happened again. A cast of talented, experienced actors have gone and bunged up their American accents. And this play - The Man Who Had All the Luck, by Arthur Miller - is the least of it. In the past few months alone we've had plays by David Mamet, Joan Didion, Steven Adly Guirgis and John Patrick Shanley. We've had Hairspray and Jersey Boys and - oh my - Gone with the Wind as well. And too often it's simply painful to listen to these characters talking, chewing their lines like toffees as they try to speak from the backs of their mouths.

A lot of West End theatre is American, but I'm not sure if I can take the assault on my ears any more. I'd love to see the Peep Show star Robert Webb in Neil Labute's Fat Pig at the Trafalgar Studios this month. But I simply don't want to sit through a couple of hours of accents more suited to Donald Duck than contemporary Atkins-dieting urbanites.

Where does this accent failure come from? “It's difficult not to stereotype when it comes to America,” explains Mary Howland, an accent coach at the London Academy of Music and Dramatic Art (LAMDA). “A lot of it is perceptual. There's an idea of Americans as brash, enthusiastic and confident. Going into an accent that is very enthusiastic, they tend to go too far.” Once an actor has the accent between their teeth, they seem to have an urge to shake it all about.

Rowland notes that the difference between Hugh Laurie's excellent American accent on the US programme House, and the dreadful attempts at it by Emma Thompson and Kenneth Branagh over the years (their 1991 film Dead Again is a standout), is that Laurie doesn't make a song and dance of his accent, while Branagh and Thompson are “constantly showing it to you”.

The technical demands of the accent itself are complex and - contrary to received wisdom - subtle. Among countless other differences, RP (received pronunciation) English is an up-and-down accent, far more singsong and “from the head” than American voices. The latter are rooted “in the body” and tend to occupy the lower end of the pitch range. According to Howland, switching the singsong for something stronger and flatter requires “going wide” - broadening out the tongue, rather than moving it up and down. While the British use pitch to stress syllables, Americans use something harder, more like a jab of volume.

But, as the coaches point out, these are actors, not phoneticians. The context of the accent, and an engagement with the character, is crucial. Penny Dyer is a veteran accent teacher whose recent productions include Mamet's Speed-the-Plow (Old Vic). “I don't just work on an accent,” she says. “It's a landscape - the geography, the history. We talk about the people and origins. We get there through the imagination. The minute you make an accent technical for actors they switch off.”

The musical Chicago boasts some of the West End's firmest American voices. Unlike many of her colleagues, the show's chief dialect coach, Judith Windsor, is American. What's her secret? Being American helps, she says. Also, the Chicago producers make a priority of the accents, and are “always willing to give the time it takes”.

She might see whoever is playing Roxy, Velma or Billy Flynn for up to 12 one-hour sessions at a time, until they get it right. Individual sessions like this are rare in theatre here. “British actors are very often left to their own devices,” Windsor says. “People tend to think the American accent is easy to do. That is not the case. In fact, it is one of the most difficult. People don't look into how the nuts and bolts of American is actually spoken.”

Learning an accent is often more visceral than technical for the actor. Andrew Buchan, unlike several of his castmates, did an admirable accent in The Man Who Had All the Luck at the Donmar last month. He went with his gut instincts, he says. “America is such an open and sweeping land,” he says. “There's that saying that Americans don't apologise for anything - they hit the last word in the sentence, with drive.”

It may be a stereotype but it put Buchan on the right track. Growing up surrounded by Americana in TV and films from childhood has only helped him. “American should be at our fingertips, I don't understand how people can find it so hard.” Indeed. But sometimes, an actor just can't do accents in the same way that some actors just can't sing or dance. “Even if you're the best actor in the world,” says Buchan, “some people just don't have an ear for accents.”

Perhaps they shouldn't be cast in American plays? Just a thought.

Jersey? No, it's Joiii-zeee...

Jersey Boys

(Prince Edward, continuing)

Yep, they handle the songs a treat. But wherever these boys are from, it sure as hell isn't

New Jersey.

Swimming with Sharks (Vaudeville, 2007-08)

Helen Baxendale's American accent held up when onstage with Christian Slater. Whenever he left, though, her diphthongs would bob precariously. Irish? Northern English? Midwestern? Answers on a postcard please.

Glengarry Glen Ross

(Apollo, 2007-08)

Jonathan Pryce, who starred as washed-up salesman Shelley Levine, is a wonderful actor. But, c'mon, he's about as American as a cricket bat.

Shadowlands

(Wyndhams, 2007-08)

The usually sensitive Janie Dee laid on the accent with a trowel playing Jewish-American poet Joy Gresham. Too much already!

The Royal Family

(Haymarket Theatre Royal, 2001) Judi Dench swore off playing Americans after her turn as a theatrical matriarch in this Twenties comedy. “The cast's energy is uneven - and so are their accents,” declared The Independent on Sunday.

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