Designers working together? Whatever happened to good old-fashioned cut-throat rivalry?

Fashion isn't renowned for its friendliness: from Prada-wearing devils, to mobile-phone-throwing supermodels, bitchiness abounds. Designers may, on occasion, join forces – but this will usually be with high-street giants (Celia Birtwell for Topshop, Viktor & Rolf for H&M, etc), not with each other.

So when the Scandinavian yummy-mummy label Day Birger et Mikkelsen asked for a little design help from its friends for the new diffusion line, Day et Friends, fashion-pack pundits were astounded. One simply can't imagine Karl Lagerfeld merrily relinquishing control with a "Karl und Pals" line...

Yet that appears to be exactly what Day designer Keld Mikkelsen has done and each season will see a new – external – designer produce the Friends collection, according to their own interpretation of the label. The first of these, which goes on sale early next month, is designed by hip London duo Preen, aka Justin Thornton and Thea Bregazzi.

Preen launched a decade ago on Portobello Road, and has since become one of Britain's hippest labels – so much so that last year the label did what many a successful fashion Brit-packer has done and defected to New York for its catwalk shows.

Fans of preen include Kate Moss, Chloë Sevigny, Amy Winehouse and Gwyneth Paltrow, with Arctic Monkeys sporting the label's menswear. As with every hip young label worth its salt, Preen has created a collection for Topshop. The range, which went on sale last autumn, focussed on sleek, body-con dresses and exaggerated silhouettes.

Inviting the super-cool label – which Thornton and Bregazzi describe as "pure rock'n'roll" – to act as guest artists to Day, best known for its "hippie luxe" boho pieces, inspired by vintage and ethnic looks, is a little akin to David Bowie joining Bing Crosby for a duet, or Jay-Z headlining at Glastonbury – not wrong, exactly, but a little left-field. Mikkelsen claims this jarring juxtaposition between Day and its first Friend was deliberate...

"We were looking at different designers," he explains, "but Preen caught our interest from a very early stage. Everything going on in their world right now is exciting: how fast the brand is growing and the direction it is taking, to the silhouettes and shapes... The Preen signature style is very different from the Day look, so the collaboration promised an interesting clash of styles."

The resulting collection is described by Mikkelsen as a "younger" line than Day proper, featuring clean silhouettes, in a minimalist monochrome palette, with the focus on the fabrics and execution of the clothes. Key pieces include a tulip-shaped white skirt with deep front pockets (far left), a bubble-hemmed white dress with voluminous sleeves (left), and a very on-trend short playsuit with a tie-waist(above) – all young, fun pieces bound to appeal to the fashion-savvy Topshop crowd, if not the traditional Day Birger et Mikkelsen customer.

Expanding the label's appeal this way was deliberate – Mikkelsen sees the Friends line as separate, yet connected, to the main brand, explaining that "the idea is to create something very different from our main collection, but at the same time capturing Day's DNA. To invite other designers we find inspiring to interpret Day their way, is a great way of offering our customers added value."

For their part, Thornton and Bregazzi see Preen's involvement in a more business-like fashion: "It was a design consultancy. We were approached to give Day a modern development, and to attract a new client." The duo describe that new client as "a modern, urban woman – independent, chic, fun and active. The attitude was a 1960s Twiggy."

These buzzwords, and the definite monochromatic 1960s look of the collection, are certainly very different from the traditional Day branding; and Mikkelsen's own style inspirations certainly don't tally with the minimalist ideals of the Friends pieces: "Day collections are based on 'ethnic', 'modern' and 'classic'. For autumn/winter 08, artists such as Egon Schiele and Gustav Klimt were the inspirations," says Mikkelsen.

Perhaps surprisingly, such clashes of style didn't lead to stand-offs in the studio, pitting designer against designer in a real-life Project Runway. Mikkelsen claims to have been pretty much hands-off, allowing Friendsto remain, well, friendly: "We worked out a brief, but Preen were free to adapt their own ideas to the Day concept."

The resulting collaboration – or interpretation – is, despite the differing approaches and aesthetics, surprisingly simpatico, and certain to woo a new following. But unfortunately for Preen and Day fans alike, fashion is fickle, and those front-row friendships rarely last. By next season, Preen will have returned to their own studio, and Day will have a new best friend.

Day et Friends featuring Preen is available at selected stores from mid-July. For stockist information, call 020 7432 8088