The reason I’m researching underwear is that, having lived in France for five years, I want to be able to walk into a room and not be immediately recognised as an English woman. I want the indefinable and alluring chic that French women have. Underwear is a lot more important to French women than it is to your average English girl, or at least this average English girl. Prior to moving to France it really wasn’t an issue I had thought that much about.

French women don’t mind spending money on underwear. A woman I meet in Paris who has been a mistress for 15 years says she devotes 90% of her salary to it.

I begin at Galeries Lafayette, one of the biggest department stores in the world. The first person I accost is English. “It’s unbelievable,” says Samantha, who is from Hull. “I used to work at Selfridges and we had about a third as much space dedicated to underwear.”

Historically, the French have always been ahead of the British in terms of underwear. They were wearing knickers way before us. In the 16th century Queen Catherine de Medici introduced them as she preferred to ride side saddle to show off her legs and needed to protect herself from the elements.

The Brits still thought panties scandalous and only fit to be worn by prostitutes. Knickers weren’t really accepted until the 19th century when the Victorians seized upon them as a good thing. The brassière was invented by Herminie Cadolle, who exhibited it at the 1900 Great Exposition and became a fitter of bras to the elite of the day, including Mata Hari.

I speak to some other shoppers and ask them what they look for. “I buy nice underwear in order to look sexy when I take my clothes off,” says Claire, a 42-year-old Parisienne.

“It’s what I put on next to my skin, so it has to be perfect,” says Julie, 28, from Amiens.

The changing rooms are sumptuous. There are two buttons on the wall; one summons an assistant to help you with sizes and styles. The other changes the light inside the cubicle from day to night, so you can see at the flick of a switch how your new bra will look when your lover undresses you. I spend at least two happy hours flicking the switch on and off in several ensembles. I wonder briefly what B’s tastes in underwear are. Then it’s back to work.

I visit the doyenne of French lingerie, Chantal Thomass, in her extravagantly decorated apartment in Paris’s exclusive sixth arrondissement. A striking figure with a sharply cut bob, she always wears black — on top that is. Underneath, she tells me, she wears all manner of colours.

Her apartment is reminiscent of her underwear collections; lots of pink, yards of stunning fabrics and patterns. I sit on a vast cream sofa in her drawing room which has two full-length windows and a chandelier.

Chantal tells me that British women actually buy more underwear than French women, but it’s cheaper and more likely to be from Marks & Spencer. “They don’t have the same culture of sophisticated lingerie that we French have,” she says, lighting up another of about 10 menthol cigarettes she smokes throughout our interview.

“Of course French women demand comfortable underwear, but it must be well designed, made from high-quality materials and enhance the way they feel about themselves. Lingerie is fundamental to the way a woman feels. If your underwear isn’t right, nothing else works.”

This might sound ridiculous, but a lot of women I speak to on the subject agree. Sophie Tellier, a former Miss France, says underwear is essential to her. “For me, if you’re a beautiful woman you have to start with beautiful underwear,” she says, looking predictably stunning in the drawing room of her new apartment. “If I start with a good base I know I will dress well as I can’t put ugly things on top.’

The actress Elisabeth Bourgine agrees: “Underwear sets the whole tone for the day. If I want to seduce, I wear a certain ensemble, if I am filming and have to be comfortable, I wear another,” she says.

One woman I met at a party recently told me she has a friend who has 365 sets of bras and knickers; one for every day of the year. How on earth would you choose which one to wear? And where on earth does she keep them all?

At Chantal Thomass’s shop on the rue Saint-Honoré I pick out some underwear in pink and black gingham with lace. The minute I put it on I finally understand what all the fuss is about. The first time you try on a properly cut set of underwear you understand how much it can do to enhance your body. I look totally different. The underwear I have been wearing seems shabby and unstructured in comparison. My whole body takes on a more defined and sexier shape. This is incredible. I need to try some more.

Délphine, the sales assistant, brings me in several things to try. It is such fun, like being given access to a classy dressing-up box. Maybe the lighting is cleverly arranged but I feel about twice as sexy as I have in years. Among the most stunning examples is a leopard-print corset with G-string and stockings. I ask the sales girl if I’m not a little bit too old for this get-up. Délphine looks at me quizzically and says there are no age limits. I remember Chantal telling me that underwear has become a fashion accessory that you can wear it as clothes. “For example, if you put on a corset with a pair of jeans and a shirt that you leave slightly open, you are dressed for anything,” she says.

I decide she is undoubtedly right and that a leopard-print corset (with suspenders of course) is an invaluable part of any girl’s wardrobe. Délphine even says she’ll throw in the leopard-print stockings for free.

According to Délphine, French women who live abroad often come home to Paris to stock up on underwear. “They can’t find the same elegance and sexiness away from Paris,” she tells me. “They come here and bulk-buy before going back. You have to remember that for French women underwear is the key to their self confidence.” As I hand over almost the monthly average salary for three ensembles, I wonder briefly who these rich French women bulk-buying might be.

As I wave goodbye to Délphine, about 20 Japanese women force their way in, giggling hysterically. They will probably spend more than the GDP of a small African country within minutes.

Sabbia Rosa opened her boutique in Paris’s sixth arrondissement in the 1960s. “It was at the height of the feminist movement and people were burning their bras, not buying them,” she says. “They would laugh as they walked past the shop, but I have always believed lingerie is fundamental to the French female psyche, and in the end I made it.”

Sabbia is a grandmother but looks more glamorous than most teenagers with her incredible curly thick hair and cat-like green eyes. “Life is tough for a woman nowadays and I think beautiful underwear gives her something nothing else can,” she says. “Confidence and assurance — even if she alone knows that beneath the business suit lurks red silk.”

Sabbia’s prices are not easily affordable. A silk slip will set you back about a very substantial amount, and then of course you have to have the matching knickers. This is the one thing every single French woman I spoke to agreed on. Underwear must be matching. When I told Christine, a French friend of mine, that I was astounded by the fact that the French all wear matching underwear she said: “Is there any other kind?”

Odile Morvan, store director of underwear at Galeries Lafayette, says underwear is all about seduction. “But for a French woman this can mean the seduction of herself, to feel good about herself and happy in her skin.”

Of course they also wear underwear to seduce, just like the rest of us, but according to Sebastian, a French friend of mine who lives in London, one of the fundamental differences between French and English women is that when an English woman wants to be seductive she will show her underwear. A French woman will not.

“It’s so tacky,” he says. “They will have a bra-strap showing really obviously, or their G-string riding up above their jeans. A French woman will of course also show a little, to give a hint, but this must never look forced or sluttish. It must look like it is an accident, otherwise it’s not attractive.”

My friend James tells me that another difference between French and English women is that French women love to unwind in sexy underwear. “They’re fantastically into it,” he says. “You can be lying on the sofa on a Sunday morning, as hungover as anything, and they start prancing around in the sexiest of outfits. An English girl would be there with her tracksuit bottoms and Gap hooded top.”

Everywhere you go in France there are smart underwear shops, even in the small town where I live. I used to walk past thinking, why on earth would you bother spending £80 on a pair of knickers and a bra. Now I wander past thinking, have I got time to try them on?

The transformation into a French obsessive seems to have happened imperceptibly. When I first moved here I would wear a pair of cotton knickers with whatever bra came to hand. Don’t ask me how, but this has all changed.

Suddenly I find it impossible to break the rules, to wear non-matching underwear. I would as soon wear odd shoes. I am now always late going anywhere as it is essential I leave the house in matching underwear. So if I have decided on a bra that will go well under whatever outfit and I can’t find the matching knickers I have to start all over again. I also spend more on a single set of underwear than I would have done in a whole year at home. An M&S five-pack is a thing of the past. What has happened to me?

There is however one notable exception to the French mania for underwear: Inès de la Fressange. “I only ever wear white cotton,” the fashion diva says. “To me all this lace is quite pathetic, it’s just something women use to wake their husbands up when they have nothing else to offer. It’s not for me. And I’ve been happily married for 14 years.”

I leave Sabbia’s shop and walk to my hotel, which is in the same street. The receptionist hands me a cream envelope. On the front is written in black ink Mme Frith Powell. Inside is a postcard of a Toulouse-Lautrec painting of a girl sitting half-naked on a floor, her stockings hanging loosely around her thighs. It reads: “I hope you had a successful trip and I look forward to seeing the fruits of your research one day, Yours B.”

I get back on the train later that evening after two days of deep immersion in underwear. Of course I am wearing my new Chantal Thomass smalls. Not the leopard-print ensemble which really would be a bit much for a train journey, but the pink gingham. You never know who you might run into on a train and, as every French woman knows, your underwear can make or break you.

© Helena Frith Powell 2005

Extracted from Two Lipsticks and a Lover to be published on Thursday by Gibson Square at £12. Copies can be ordered for £10.80 with free delivery from The Sunday Times BooksFirst on 0870 165 8585