There is a battle going on for Britain's breasts - or at least, the bras that support them. Within the past decade, bras have become big news. Retailers now talk about customers having a 'bra wardrobe' containing the average woman's sports bras, fashion bras, T-shirt bras, nude bras, gel bras, sexy bras and more. Bra executives talk of marketing them as a lifestyle and special-occasion item.

Impressive figures the underwear market is projected to be worth £2,600 million in 2008, with the luxury sector performing particularly well

This bears no resemblance to the comparative dark ages of the early 1990s when women could either brave the darkened windows of Ann Summers or the more matronly offerings of a department store such as Marks & Spencer. Back then, most bras could double as an ogre's slingshot or carried warnings about standing too close to a fire. Women weren't drowning in choice the way they are now, so it is hardly surprising that on average, we only bought one bra a year.

Two events in 1994 dramatically altered the history of the bra: Eva Herzigova in the 'Hello boys!' Wonderbra advert and the opening of a small shop in Soho called Agent Provocateur, with a scantily clad dummy in the window. Darkened windows were history. (Last year Agent Provocateur was sold for £60 million, and this year it opens its 38th store; its range now extends to vamp shoes and whips.) Today there isn't a high street in the country without 'come hither' lingerie shops vying for our attention. We now buy on average four bras a year - the most per head in Europe.

The upmarket Russian lingerie company Wild Orchid, which has a turnover of £50 million with 216 stores throughout Russia and Ukraine, made its British debut in the summer at the Bluewater shopping centre in Kent, followed by a shop on Kensington High Street in London. A third shop is planned to open in April on Oxford Street, selling its own brands alongside Christian Lacroix, Roberto Cavalli, Dior and Alberta Ferretti. Wild Orchid is the first Rusian retail chain to break into the European market. The Italian company Tezenis opened last year on Regent Street selling competitively priced underwear (it has 100 stores in Italy and is aggressively expanding throughout Europe).

Even Marks & Spencer has ditched the matronly reputation (though if that's what you are looking for, it is actually still there if you dig around) and has gone largely fashion-oriented with seven 'fashion forward' brands. Soozie Jenkinson, the head of lingerie design at M&S, says, 'Design has been driven by high-street trends. Lingerie has become a fashion accessory.' M&S sold 20 million bras in 2007 - 10 per cent more than in 2006.

At the top end of the luxury market, most brands have expanded their lines into lingerie. The latest to launch is Stella McCartney, whose range went on sale in February in Harvey Nichols, and at Liberty's new luxury lingerie boutique. Her organic hand-stitched designs have adopted the current trend for a coy, schoolgirl take on sexy, so they have names such as 'Ava Swinging' and 'Dolly Snogging', costing between £90 and £155. As with branded luxury accessories, such as sunglasses and make-up, lingerie is a lower price point that makes buying into the big-brand experience affordable. Designer bras are now the new handbags.

In 2007 the luxury lingerie brand Myla (which was once a small player, but like Agent Provocateur is now owned by a consortium of private equity and venture capital firms) saw its sales increase by 67 per cent. The business of bras is increasingly a place of takeovers where small labels struggle to grow and compete.


Catherine Bailey in Agent Provocateur's campaign

There is an exhausting energy in bra land. Maybe it is because of the statistics. In 2006 we spent £2.48 billion on our lingerie in Britain. Katrin Magnussen, a senior retail analyst of the underwear retail market and editor of the latest Mintel report, says, 'We estimate that the overall underwear market, which was £2,530 million in 2007 (up by 2.6 per cent), is projected to rise to £2,600 million in 2008. The growth is definitely set to continue. A bra is not too expensive a thing to buy to make you feel good when you're feeling the credit crunch. If you go into value stores or somewhere like Primark on Oxford Street, the prices there are low enough to make anyone feel like they've won the lottery.'

Brands such as La Senza (8.2 per cent market share, 186 stores, sales up 12.4 per cent to £98.2 million), Ultimo (10 new stores to open in 2008) and Agent Provocateur have up to four advertising campaigns a year. Michelle Mone, Ultimo's Glaswegian founder, explains the brand's fast turnover: 'We do five new collections every six months, so our customers always get to see something new.'

So why the rise in demand? Everyone interviewed for this piece universally attributed one factor as Trinny and Susannah and their 2001 What Not to Wear television show, which transformed the nation's attitude to underwear. Their relentless assault on laughably awful underwear shamed many of us to take a long hard look at our boobs and realise we, too, needed to go shopping. Wearing the right bra size miraculously made us feel more confident - giving us a better posture and making us look slimmer. So why Trinny and Susannah haven't branched into bras is a mystery, particularly since their Magic Knickers range, despite resembling semi-surgical post-operative apparatus, has been a huge hit.

It is something the queen of the expert fit, Rigby & Peller (by royal warrant since 1960), has always known: 80 per cent of us wear the wrong size bra. But being properly measured isn't the end of the story. Women also want to wear something fun, sometimes flirty and often fashionable.

So what else makes women go shopping for bras? Franceska Luther, the creative director of Myla says, 'Women today exist in a "have it all" society. We strive to be the best we can be - both at work and at home. We want the most fabulous clothes, the best beauty products and this must be complemented by the finest lingerie.' Even if we can't afford £89 for a bra, we are seduced into buying it anyway. British women's relationship with their underwear has moved on from being about functionality. They now buy into it for its feelgood factor.

The headline news that the whole bra industry is focused on is that the nation's breasts are getting bigger. Where once the average British bra size was 34B, it is now 36D or 38C or 34E depending who is talking. Marks & Spencer has just launched a J cup. Whether it is due to the fattening of the nation, or the boom in cosmetic surgery, or mystery hormones in the water, no one can say for sure. It is good business for Bravissimo, a bra success story dedicated to larger cup sizes D to K. Set up in 1995, it now has 16 stores, a strong online presence, and a £37 million turnover in 2007. Ultimo, the £45 million company known for its mid-price bras, is also launching a new G cup and has several bra 'inventions' (backless and frontless bras) specifically targeted at bigger cups due out this year. 'When you realise that could be the weight of your head in each cup, that's a lot of engineering,' comments Michelle Mone, whose company first attracted attention when Julia Roberts wore its gel bra in Erin Brockovich.


Petra Nemacova models for La Senza

Even Agent Provocateur, better known for its dainty bras, now goes up to a 38F and does a maternity bra. La Senza reports a 30 per cent increase in sales of the DD to the F cup. So much for the size zero debate. What we wear under our clothes is a separate story that has nothing in common with the prepubescent frame seen on most catwalks.

Online sales are a growing force. Figleaves.com, a privately owned solely online company, dominates almost any Google search you care to attempt. It has just hired Julia Reynolds, formerly a buying director for Tesco's £1 billion clothing business. Figleaves's turnover is £30 million. It is considering using celebrities, launching a coffee-table book campaign and opening a central London shop focusing on 'luxury that is affordable'. Every brand you can think of can now be bought online. And there's plenty of energy in online communities delivering the latest lingerie news on websites such as louloulovesyou.co.uk, misslalasboudoir.co.uk or knickersblog.com.

Perhaps the biggest criterion behind the boom in bras, and the one that is hardest to quantify, is that women aren't the same as they were in 1994. These days they know all too well what they want, they're going to ask for it and they're certainly not waiting for a man to buy it for them. There aren't many blushes happening in the fitting-room. Women don't feel the need to look good for anyone but themselves and if someone else appreciates their lingerie, then that's a bonus. Whether it is sexual confidence or social change, it has had a big impact on the psychology of the average bra shopper.

The bra market is fully aware that it needs to differentiate its brands - and it uses celebrities. Wonderbra has the queen of burlesque, Dita von Teese, Ultimo has Sarah Harding of Girls Aloud, La Senza has the tsunami survivor and model Petra Nemacova, Marks & Spencer has Noemie Lenoir, Agent Provocateur has Catherine Bailey, Victoria's Secret has model mum Heidi Klum. Otherwise bra brands use none-too-subtle advertising to corner our attention in a near saturated market.

A high-budget and entertaining approach can be seen on Elle Macpherson's Intimates website (turnover £40 million). Here you are offered an artful voyeur experience. A keyhole appears on your screen and turns you into a voyeur peeping at the latest collection. It is super-stylish with a vintage tone and a retro saucy theme that is echoed across many brands. Agent Provocateur offers its own combination of The Erotic Review mixed with 1950s Hollywood, with hints of bondage and suggestive poses. There's even the actress Maggie Gyllenhaal gazing vulnerably at the camera, in a recent collection, tied up, wearing 'Sophie'. Basically anything goes, and we seem unshockable.

But whether it is the advertising or the celebrity that initially corners the customer, it is in the shop that the majority of transactions take place. Joe Corre of Agent Provocateur thinks that his pink boudoir shops are 'where people get our whole experience, with people who know what they are doing.' Theo Paphitis of La Senza, which is currently expanding its 186 stores into Europe, believes that details such as wrapping the purchase in tissue paper and putting the receipt in an envelope 'even if they've only spent £6, make it an experience.'

So despite every sign of a recession, can women's seemingly insatiable appetite for lingerie make it recession-proof? 'Some women may trade up for investment bras, and some may trade down for something cheaper. If you only pay £5 for a bra then it's a guilt-free treat,' Mintel's Sally Bain comments. But with so much evidence of competitive pricing, Michelle Mone is adamant that 'the bra that does the job will survive whereas the bra that is just a brand will disappear.' Achieving growth is definitely going to be tougher going. And the jostling between product innovation, price, fashion and functionality within a bra that also needs to make us feel and look good at the same time is a fine balance. One thing's for sure, there is no going back; this is a battle that will run and run.