The frill of it all: the high fashion concept of underwear is not just for pop stars, it's also popular on the high street

• Lady Gaga, pictured left, made it a trademark look

Donning one's undies to go out in public has been on fashion's radar for a while now. Jean Paul Gaultier and Dolce & Gabbana have been doing it for ages, and designers including Fendi, Christian Dior and Marc Jacobs have caught up this year. Madonna did it back in the early nineties with her conical bra and corsets in the stage costumes worn during her Blonde Ambition tour - and of course Superman has been wearing his pants outside his trousers for decades.

However, it's only recently that the trend has made the leap from racy, lacy scraps of fabric on show to underwear itself getting the high fashion treatment. Putting traditional underwear on show is all very well and shock-worthy, but shapewear brand Triumph International have made it their mission to redesign the underwear itself - and you wouldn't want to cover this stuff up.

They invited fashion students across the world to re-define traditional lingerie, under the theme "Shape Sensation". Some 27 finalists from across the globe have been selected, and a winner - whose design will go on sale next year - will be crowned during London Fashion Week in September. All the final designs have been modelled by Helena Christensen and caught on film by Scottish photographer Rankin, both of whom are sitting on the judging panel.

The finalists have created designs using non-fabric materials ranging from plastic to wood. One has wings; another is like a technicolour suit of armour; yet another features balloons filled with coloured dye which marks the fabric of the underwear as the balloons burst. These creations demand to be on show, but this is not about wearing a silk slip as a top or showing the lace on your stockings. Here, sex comes a distant second to fashion.

The UK underwear market was worth 2.57 billion in 2009 and is expected to increase to 2.59bn by the end of this year, according to research firm Mintel. And despite the recession, sales of lingerie have risen by 9 per cent since 2004.When women are spending this much on underwear, they want people to see it.

In the mid-nineties, luxury underwear emporium Agent Provocateur popped up, offering catwalk-worthy undies to fashion-conscious customers, claiming that until they came along, bras came in black, white, nude, or red for Valentine's Day. They were right.

Luxurious and fashionable they may be, but Agent Provocateur's undies - and the cheaper versions they inspire on the high street - are built with titillation in mind. They are lacy, silky and most definitely saucy. It has only been more recently that customers have been able to buy the kind of underwear that is fashionable enough to put on show, but sexless enough to avoid giving your granny a heart attack.

High street brands such as Topshop and Miss Selfridge now sell what they call "bralets" - a cross between a bra and a top, in deliberately unsexy fabrics such as grey sweatshirt material, which are designed to be worn on their own or over a T-shirt depending on the bravery of the owner.

Tina Warren, who runs Club Noir, the Glasgow-based burlesque club, says: "Back when Elizabeth Taylor had her petticoat on show in Cat on a Hot Tin Roof, that was considered really shocking. Today no-one bats an eye at underwear on show.

"I like to wear a corset as outerwear, but I might wear it over a shirt or a T-shirt to give it a more casual feel. And of course there's also underwear which can't be seen, but can be, shall we say, be 'perceived' like a pointy bullet bra. I think that's a great look too. The bottom line is that when you own pretty things, you just want to show them off."

Back in 2007, Sienna Miller famously wore a pair of large black pants on the red carpet, but this year - thanks to Lady Gaga - frumpy-yet-high-fashion pants as outerwear have made it on to civilian shopping lists and onto the streets. Last month, lingerie retailers reported a surge in demand for big pants, with venerable brand Rigby & Peller (which made the big briefs memorably worn on film by Rene Zellweger as Bridget Jones) recording a 15 per cent swing from thongs to larger styles this year. Online retailer Figleaves say they are now selling two pairs of fuller briefs for every G-string.

"People who want to wear the latest pair of shoes, now also want the latest bra," explains Vasilisa Forbes of high-end lingerie retailer Apartment-C.com. "When it comes to underwear, we think in seasons, just like the rest of the fashion world, and there's a big demand for high-fashion underwear at the moment.

"We've embraced the underwear as outerwear trend and so have a lot of our customers. This doesn't have to mean wearing pieces that are very obviously 'lingerie'.Lots of people like that minimal look favoured by Lady Gaga which could almost pass for outerwear anyway."

So if you're itching to put your intimates on show, what's the best way to get in on the act?

"The underwear as outerwear trend hasn't been seen to this extent this the 1980s," says Emma Dixon of Edinburgh boutique Miss Dixiebelle. "This time round, however, it is all about layering up; say the peek of a satin bra under a fitted white shirt, to create a fuller outerwear look.

"Retro brands such as What Katie Did are perfect for this trend as the nostalgic colours and shapes - 1950s bras for example - hint at soft, sweet and a little bit naughty rather than full on sex appeal.

"Research has shown that lingerie sales increase substantially, in some cases by 20 per cent, during recessions. A feel-good purchase, it makes women feel great about themselves and the 'outerwear' trend allows us the opportunity to show it off in all its glory."

There are plenty of lace bodysuits on the high street (don't forget your nipple covers, sales of which have rocketed this year) as well as high-waisted pants (wear over your tights like Sienna if you dare) and of course, bralets. And if you're feeling brave, you could go Gaga in see-through rubber. My, we've come a long way since Bridget Jones.

FROM UNDER COVER TO PROUDLY OUT ON SHOW

Just 40 years ago, a bra strap on show would have been considered scandalous. Today it's considered fashion.

The trend for underwear as outerwear kicked off in the 1980s, fuelled in a large part by Madonna, whose conical bras, girdles over her clothing and visible lace undergarments set trends across the globe.

By the Nineties, men picked up on it too, wearing their jeans slung low over their designer boxer shorts so as to display the logo emblazoned on the waistband. By the end of the decade, women had joined in - the trend for hipster trousers, set by Alexander McQueen, meant it was difficult to keep one's underwear off show. Soon, women were wearing their thongs pulled up high over their trousers.

The Nineties was the decade of "heroin chic" and of minimalism and a trend emerged for visible satin slips with thin straps. Fast-forward to the noughties and the only trends worth knowing about were set on Sex and the City. Sarah Jessica Parker's character Carrie regularly had her bra on show, and wore camisoles as outerwear.

By 2009, underwear as outerwear had taken on an almost armour-like feel, influenced by the stagewear worn by artists including Lady Gaga and Beyonc. Bras were sturdy, with simple, clean lines, designed to be worn with high-waisted trousers or skirts to show off the upper midriff.

Daring types now don large belly-button concealing briefs, inspired by 1940s-1950s lingerie. They are usually toned down with tights and a shirt or baggy jumper.

Over the last ten years, the trend has grown more and more extreme, and it shows little sign of slowing down.