From Raf Simons’ appearance at Frieze Art fair in London last year (low-key enough to suggest he was buying, as opposed to posing) to the proliferation of Céline bags spotted in any given uptown gallery, via Sam Taylor-Wood’s modelling slot in the Net-a-Porter ad campaign last season, it’s clear that any separation that previously existed between the words of art and fashion has almost entirely melted away.

It certainly seemed that way at the party COS held at the Serpentine last night.

They’ve always been close as metiers, of course: early modern portraiture can be read as an ode to the fashionable clothing worn by the wealthy patrons who commissioned the imagery, while archetypal style icons, such as Peggy Guggenheim and Talitha Getty, have surrounded themselves with creative types throughout history, funding ideas and fizzing up creativity nicely - while dressing the part, of course

'We feel that the art and fashion world have many things in common and that designers and artists can learn a lot from each other in terms of inspirations, materials and procedures,' says karin Gustaffson, head of womenswear design. 'At COS we feel that art has always been relevant to what we do, it inspires our collections year upon year with shapes, colours, cuts and silhouettes. The nature of fashion and clothing means it constantly inspires everyone – it is a key part of everyone’s life.'

Elsa Schiaparelli worked alongside the Surrealists and the Dadaists in the Thirties, rendering their ideas as wearable pieces – a lobster print dress; a hat shaped like an inverted shoe; a trompe l’oeil of hands clutching the body on a top – and creating prints with the likes of Jean Cocteau. Then there was Yves saint Laurent, who turned de Stijl artist Piet Mondrian’s cubes into a Sixties shift. Clearly there’s something symbiotic going on.

‘For the last decade, fashion has increasingly been exhibited as art in its own right, the most prominent example being the Alexander McQueen show at the Met in 2011,’ explains Sam Thorne, associate editor at Frieze. ‘At the same time there have been several high-profile collaborations, like Takashi Murakami working with Louis Vuitton. Likewise, there are people like Hedi Slimane who has gallery shows as an artist while designing at Saint Laurent.’

'For autumn 2013, we wanted to put products into new context and looked towards interior designs by Willy van der Meeren as well as the Arne Jacobsen’s SAS hotel in Copenhagen,' explains Karin Gustaffson. 'We also worked a lot on precision silhouettes and cocooning shapes and were inspired by the contemporary artist Lucy Orta as well as photography by the mountaineer Sandy Hill.'

And with a proliferation of fashion labels sponsoring or collaborating on art events – COS at Frieze, Louis Vuitton’s partnerships with the likes of Grayson Perry and latterly, Ben Eine, Marc Quinn’s florals on a Lucien Pellat Finet cashmere sweater – comes a relatively new idea that those interested in one would automatically be interested in the other.

It doesn’t sound that radical, but it’s a fairly new theory. Gone is the separation between square-haired art critic in chunky jewellery and ugly shoes, and loud-mouthed fashion Barbies arrayed in sheeny satin. The look of both now, at the industry’s most modern and exclusive edges, is something more refined and less extreme. And it’s no coincidence that Phoebe Philo’s references are all black and white photography and abstract paintings.

‘Why's this happening? The optimistic answer is that this is all about broadening the restrictive definitions of what 'art' and 'fashion' can be,’ adds Sam Thorne. ‘That is, realising they're both bound up with issues of display and being in the world.’

That isn't to say everyone in the art world has signed up to its new-found fashionability though. ‘The cynical answer is that the same collectors buy both,’ Thorne continues. ‘Consumer brands sponsor events at museums; the fashion world gets its intellectual cachet via a presumed proximity to the art world, while the art world gains a frisson of desirability.’

What do you think? Click the gallery to see some of the party's guests.