One month before Christmas, the online fashion retailer Net-a-porter.com launched its Boutiques section, comprising seven categories including Essentials, Vacation, Party and one striking new contender – Loungewear. “We extended our category offering with the introduction of loungewear due to increasing customer demand,” confirms Holli Rogers, the website’s head of buying and retail.

At Browns boutique in London, a similar story was unfolding. “Luxury loungewear is a category that we’re looking to develop,” says buying director Erin Mullaney in December. “I think mainly because of the economy and the way people are shopping now; people are staying home a lot more and are looking for things that have value for money.”

As staying in becomes the new going out, wardrobes are evidently adapting. Rising membership of online DVD rental services and increased sales of smoked salmon attest to a growing number of dinner parties that make that wardrobe stacked with glittering ballgowns rather obsolete.

From left: Halston autumn/winter 2008; Philosophy spring/summer 2009; Burberry s/s 2009; Halston a/w 2008

Luxury loungewear, a discreet style that bridges the gap between structured ready-to-wear and slouchy sleepwear, is rapidly becoming the most appropriate sartorial solution in the current climate. “Everything is softening,” says Mullaney. “It’s not just about going out to fabulous parties. People are having dinner parties at home again, quiet cocktails and things. It’s not so over the top and decadent and in your face. I think the clothes are reflecting this.”

At Juicy Couture, a pioneer of the loungewear trend, the statistics speak for themselves: founders Gela Nach-Taylor and Pamela Skaist-Levy confirm a 48 per cent increase in sales for the first half of 2008. And that’s not all. From one store, opened in 2007 on Rodeo Drive in Beverly Hills, Juicy Couture has expanded to 61 shops across the US. This year a new shop will open in London and another 11 in Asia, to bring the international tally to 27. If you’re looking for a safe sartorial bet in the uncertain year ahead, this is it.

Loungewear’s success is partly due to rebranding. No longer a market dominated by slouchy velour tracksuits, luxury loungewear (note the emphasis) is bigger, better and, dare we say it, dynamic. The hooded sweat top has been replaced by the “blanket coat” – an upmarket outdoor version of the dressing gown – at Michael Kors, Halston, Rick Owens and Stella McCartney; the jogging bottom has become looser, more luscious and even rather stylish in the form of a harem pant; accessories have a more luxurious edge while still maintaining structure, such as Jimmy Choo’s fleece-laced boots and the Ugg boot that is made from leather but retains a sheepskin lining.

Luxury loungewear is even viable in the office. Oversized cardigan coats are steadily rivalling the cumbersome and costly overcoat and are just as smart; cashmere and merino are becoming ever sleeker and more decorous; while accessories such as lined leather boots combine formality with discreet comfort. “There is no reason why something that you feel comfortable in at home can’t be worn outside, as long as the pieces are made in beautiful fabrics and cut in an interesting style,” says Moise Emquies, founder of the casualwear brand, Splendid. “A new take on the motorcycle jacket or pea coat, cut in super soft fleece, are two examples of new styles that have done amazingly well.”

The harem pant is representative of the trend’s evolution. Once an item relegated to bikram yoga classes, last year it made its way on to the catwalk at Philip Lim, Ralph Lauren, DKNY and Giorgio Armani as a luxurious evening wear alternative. “We touched on harem pants because we thought it was really important to,” says Browns’ Erin Mullaney. “We bought into them not in a day way but in an evening way, translating them into a tuxedo/smoking jacket style; relaxed but still dressy.”

So while buyers wave goodbye to conspicuous consumption, relaxed chic is picking up the slack – and the big brands are catching on quick. Burberry Prorsum’s love affair with new wave grunge can find no better bedfellow than luxury loungewear, while Halston tuned into the zeitgeist with its louche jersey separates, becoming the beacon brand for Net-a-porter’s Loungewear boutique.

Yet one question still niggles. When we’re meant to be buckling down and working hard, is the growing emphasis on lounging a trifle hard to swallow? If taken literally, perhaps. Clocking in at 9am in a cashmere top and trousers, primed for that post-prandial nap, hardly creates the right professional impression. But loungewear no longer conforms to its original parameters – just look at its regular prefix, “luxury”, for proof. A cashmere coat, or a sheepskin-lined boot is not inextricably linked with lazy weekends on the sofa; a silk harem pant is easy and unstructured yet lovely enough to take out on the town; patent travel slippers work in the airport VIP lounge and on the plane. In other words, the success of loungewear, 2009 style, is its increased versatility.

“In the current economic climate,” says Bonnie Takhar, president and chief executive of Halston, “investment dressing will prove to be key, and the concept of luxury loungewear – which allows the wearer to dress luxuriously yet discreetly, season after season – is the right fit.”

Nicola Copping is the FT’s deputy fashion editor