There is an episode of the Simpsons in which, in an effort to convince his young patients of the importance of dental care, Lisa's orthodontist heaves out a book entitled The Big Book of British Smiles. The punchline, of course, is that the British are famed for possessing teeth that are so gnarled and yellowed and gappy that they can frighten children across the world into obsessive dental hygiene. Our teeth have a reputation that has caused us to be the butt of many a joke in America and beyond; in Mexico, for example, bad teeth are known as "dientes ingles". And, for a while, we Brits were faintly proud of our unseemly smiles. Not for us the white picket fences of American mouths; ours were dry-stone walls: craggy, uneven, weathered.

But then something changed. Braces became a rite of passage for most children, we bought electric toothbrushes and whitening toothpastes, stealthily beginning the pursuit of the Hollywood smile. This week, conclusive evidence that cosmetic dentistry had moved beyond the realm of glamour models and soap stars came with the publication of photographs of the chancellor, Gordon Brown, showing his peggy, stained teeth to have been replaced by a perfect chorus-line of pearly whites. The fact that even the champion of prudence appears to have shelled out several thousand pounds to perfect his smile proves that great gnashers have finally arrived on these shores (though a Treasury spokesman yesterday would say only, "He looks after his teeth like anyone else. Any work he has had done over recent years has been for reasons of dental health and not image.") But how, precisely, did we get to this? When did Britain, of all places, start flashing its minty-fresh, brilliant white smile?

Of course, much of the new-found British affection for a shiny smile stems from a more general infatuation with America. In the US, where there is no national health service and all dental procedures must be paid for, cosmetic dentistry has long been popular, extending in more recent times to include at-home procedures, as the line between health and beauty products becomes increasingly blurred. In 2001 Procter & Gamble invested $45m (£26m) in telling its American consumers about at-home whitening, prompting the market to increase tenfold by the end of 2004. P&G's Crest Whitestrips kick-started the trend, prompting rival toothcare brands such as Colgate-Palmolive and Unilever to launch similar products. In 2004, Euromonitor International reported that sales of whiteners in the US sat at around $164m in 2001, and grew by 98.9% in 2002.

Now the industry has set its sights on the UK. Research from the British Dental Health Foundation shows that almost one in two people would now consider having treatment purely for cosmetic reasons; meanwhile the British Dental Association is unable to provide precise figures for the number of people buying whitening products, having dental whitening or other cosmetic dentistry procedures, but what is certain is that we buy electric toothbrushes and whitening toothpastes, invest in Crest Whitestrips, teeth-whitening, veneers, with an increasing willingness. Whitening toothpaste alone took a 21% share of value sales in 2003, compared with 10% in 1998. (One hindrance to the growth of the market so far has been EU legislation, which regulates levels of hydrogen peroxide in whitening agents. The EU is yet to pass legislation allowing levels of 6% hydrogen peroxide, which is used in whitening brands such as Whitestrips found in the US.)

The historical reasons for our national straggly-toothedness are various. "In the 18th century," notes William Leith, author of British Teeth, "posh people actually had worse teeth, because sugar was a rich person's thing, therefore bad teeth didn't have the same social stigma." But the real descent into decay came with industrialisation. "We were the first country to be industrialised on a big scale," he says, "which meant people moving from farmland to the cities to work in cotton mills and factories and this meant that a lot of people's diets in the 19th century took a nose dive. Those working in the factories also found that they could keep going if they drank lots of sugary tea."

But it wasn't just about bad diet and access to sugary stuff. The NHS brought better dental health for the country at large, but there has long been something about being British that made attending to the state of one's teeth somehow less imperative, or even distasteful. "We have such a powerful class system, and the essence of aristocracy or gentlemanliness was not making an effort," notes Leith. "And under that came beautifying themselves. Our aristocracy are frequently portrayed for comedic effect as people who live in crumbling houses and have bad personal hygiene, people who are so self-confident and sure of their own identity that they don't have to make any effort." Consequently, the American penchant for tooth-whitening was for a long time rather looked down upon in Britain. "I believe Americans were whitening their teeth in the 50s and 60s, so there was probably the idea that it was a little bit vulgar."

No longer. Those nostalgic for Britain's traditional tombstones may trace the rot to 1995, when the novelist Martin Amis spent $20,000 (£12,000) having his teeth done. (In that same year he was given an advance of £480,000 for The Information, so it wasn't as if he was short of the cash.) Prior to this, no one in Britain gave much thought to their teeth. They chewed for us, gnashed for us, gnawed for us and in return we gave them stripy toothpaste and, if they were lucky, an occasional trip to the dentist. Amis's decision, accordingly, was treated with derision, a symptom of unpalatable vanity (AS Byatt called it a "folie de grandeur"). Privately, however, Britons began to ponder the state of their own teeth. If even our national literary icons were smartening up their mouths, perhaps the rest of us should be considering doing the same.

The celebrities, needless to say, were at it already, coinciding with a new wave of celebrity magazines that showed endless photographs of film stars, pop stars and soap stars with gleaming white smiles. "It's been going on for five or six years," says Ellie Crompton, fashion editor at Heat magazine. "The obvious ones are Noel Gallagher, Shania Twain, Celine Dion - they literally were snaggletooth yucky-mouths and I honestly don't think they would have been as successful if they hadn't had their teeth done. I think these days, if you're on TV or in the public eye, it's a given that you'll have it done." Close examination of grins over the past decade suggest that Anne Robinson, Carol Vorderman and Catherine Zeta-Jones are among the Britons who have had cosmetic dentistry.

"It's all connected to the obsession with surfaces," says Leith, "and I suppose cosmetic surgery has something to do with it. The thing is that [it] will never really satisfy people; it's not better teeth they want, it's to be a better thing, a better person. But as soon as you make yourself prettier you make everyone else look uglier, you raise the bar, and soon everyone will have white teeth. And then what do you do? But to have bad-looking teeth in an age where it's possible to have good ones says something bad about you. Gordon Brown is showing that he's modern, that he's not a stick-in-the mud. We all worry that he's more grungy than Blair, but Blair's got quite crooked teeth."

Of course, the boom in cosmetic dentistry is merely part of a more general acceptance of plastic surgery on this side of the Atlantic. Nose jobs, boob jobs, chemical peels - all have become increasingly common and acceptable in the UK. In 2004 alone, for example, the market increased by 50%. Bupa estimates that Britons have 75,000 cosmetic surgeries carried out each year, with a further 50,000 non-surgical procedures, such as Botox; having one's teeth straightened seems very minor by comparison.

According to cosmetic dentist Dr Ash Parmar, of the Perfect Smile Studios, cosmetic dentistry began in the UK around six years ago when Larry Rosenthal, the top cosmetic dentist in the US, started lecturing to dentists here. Coupled with the increased popularity of makeover shows and an increasingly positive portrayal in the press, it gained in popularity. "The main category is 40- to 50-year-olds," he says, "especially women, who feel age is creeping up on them. A nice smile can make them look 10 years younger."

There are, Parmar says, an array of different cosmetic procedures from which the dentally-challenged can choose: "Veneers, which are thin fingernails of porcelain glued to the front of the teeth - that's the Rolls-Royce of cosmetic dentistry; 10 veneers would cost between £8,000 and £10,000. A 'mini-makeover' would include contouring or polishing the sides of the teeth to make them more even, composite would fill the gaps, and tooth-whitening would cost £2,000-£4,000. Titanium implants to replace a missing tooth can cost between £1,800 and £3,000 for a single tooth. And common-or-garden tooth-whitening is between £500 and £1,000." Brown, he hazards, has probably had porcelain veneers. "If Tony Blair had had professional PR advice he would have had it done a while ago, too. He would look smarter and more presentable."

But while it might be nigh-on compulsory for the stars of daytime soap s or competitors on X-Factor, tooth-whitening is a tricky business for a politician. Blair may have faced mild mockery over his somewhat scraggy teeth, but any suggestion that he was spending too much time or money in improving his appearance would also be likely to invite ridicule. "You might say the type of politician who has their teeth done or a facelift is perhaps the wrong type of politician," suggests Leith, "because the worry that we have with politicians is that they apply a veneer rather than sorting out the problem."

As with everything from blonde highlights to tanning lotions, it is possible to take self-improvement too far. Some dentists have raised concerns about the number of patients who are requesting unnaturally white veneers or damaging their gums by using over-the-counter bleaching products bought overseas. Jodie Marsh's teeth, which were cosmetically improved on a reality television programme, are, says Crompton, an example of teeth that are excessively white. "They're far too white for her skin colour; if you wear that much fake tan and your teeth are that white you are just going to look like Malibu Barbie."

At his practice, Parmar has also noted an increasing desire to achieve the dazzlingly white LA shades. "We used to be darker than America, where they go for really white, toilet-bowl shades, but we are catching up," he says. "A few years ago, 'A1' was seen as a good colour here - it was a light, creamy-yellow colour. Now people want B1, which is a natural white, and the bleach whites which are whiter than natural tooth colour. It's against the grain of what we as dentists want, but if it's what people want then we'll do it."

It is all a long way from the days when we relied upon the NHS to care for our teeth, expecting the nearest chap with a drill to fill our mouths with amalgam as required - and for free. Britain's free dental services are in crisis - more than 2 million patients are currently unable to register with an NHS dentist - and as a result, we have grown accustomed to paying for treatment, raising our expectations accordingly. Meanwhile, as Leith notes, we may want our teeth to look perfect, but we are not much better than we used to be at looking after them - less than a quarter of us floss regularly, and one in five even admit to brushing less than the recommended twice a day. Meanwhile, the popularity of sugary, tooth-rotting foodstuffs shows little sign of abating.

"Is it right to whiten your teeth or is it wrong?" wonders Leith. "Well, it's wrong in that you're buying into the new beautification and making everyone feel bad for not having whiter teeth. On the other hand, it could be filed under stain-removing. And what's wrong with stain-removing?" He admits that he has, himself, been contemplating tooth-whitening lately. "I was really thinking about it the other day. I had terrible trouble with this tooth and I subsequently got all my teeth fixed and tarted up with white fillings ... And I thought, maybe I should have them whitened? It's a bit like, 'I drive a normal car and the last few years I've felt dwarfed by SUVs, boxed in by Range Rovers, and begun to think maybe I should get an SUV.' It's very much like that. I've begun to think, does there come a tipping point when you really should do this because it's a disadvantage not to? Maybe we've reached that point?".