Forget dainty kitten heels or towering stilettos. The sandal to be seen in this spring is the "ugly shoe": footwear that is hefty, flat and defiantly unsexy.

Seen on the catwalks of Céline, Prada and Givenchy – and on celebrities including Alexa Chung, Rebecca Hall and Lauren Laverne – the ugly shoe trend encapsulates a variety of styles, from Céline's neoprene pool sliders to sequinned hiking sandals at Prada and Marc Jacobs to luxe, floral Birkenstock-inspired sandals at Givenchy. All are united by an emphasis on chunkiness and comfort. Easy to recreate inexpensively, using rubbers and velcro, ugly shoes are big news on the high street, too.

"We haven't seen such a fast-moving trend in flat shoes for a long time," says ASOS.com senior footwear buyer Polly Thrussell. "Last year, 5% of sales in flat shoes were driven by chunkier silhouettes. This year we have planned for 60%."

"They are probably out-selling high heels," says Topshop buying director, Maddy Evans: "There is such an appetite." Whistles' take, the Maya Gladiator Footbed – four thick leather straps with a cushiony rubber sole – went on sale in January in black, white, navy and silver. Already, the silver and navy versions have sold out.

Rebecca Hall wearing Christopher Kane metallic sandals. Photograph: AGF/Rex

As the name suggests, the ugly shoe is hard to love – at first. One furry blue version, by Céline, so incensed the Daily Mail that the newspaper was compelled to ask: "Are these the world's ugliest shoes?" That said, the trend has been incubating, over a couple of seasons, among the more experimental sections of the fashion industry. For his spring summer 2012 collection, Christopher Kane sent models down the runway in metallic pool sliders. Soon afterwards, fashion bloggers and a few celebrities, like Rita Ora, started wearing a far more affordable version: plastic sliders by brands like Nike and Adidas, which cost about £20.

Last summer, style magazine AnOther proclaimed 2013 the year of the pool slider revolution, offering Céline's SS13 fur-lined Birkenstocks as a possible starting point. The editor of anothermag.com, Laura Bradley, described the trend as one that "started on the street" and was "unexpected; just like all brilliant things in fashion".

Though the head of fashion at matchesfashion.com, Natalie Kingham, had a pair of the Christopher Kane originals, "it wasn't until this summer that most people's eyes seemed to readjust to them, and that translated to sales".

"When I first saw them, they remind me of the shoes we wore in the 1990s," she says. "I remembered how popular they were then, and I thought about how well they would work with the current relaxed, sporty vibe."

Stella Tennant attends the Chanel show as part of the Paris Fashion Week Photograph: Michel Dufour/WireImage

Indeed, in a year that Karl Lagerfeld sent models down the runway in trainers at the Chanel couture show – and with trend forecasters predicting that normcore, the act of studiously dressing plainly, will be the decade's biggest fashion trend – it's perhaps no surprise that ugly shoes are being accepted.

Prada show, spring/summer 2014. Photograph: Rex

"They are often called unsexy," says Kingham "and maybe they are not men's favourite shoe for women to wear, and they can look slightly orthopaedic, but they make you look cool. They offer a lot of comfort; you can run around in them. I think they will be around for a few seasons."

Thrussell agrees: "It's a trend that offers comfort – and those are the ones that last. Since last season, when Stella McCartney-esque chunky ankle boots were a huge success, gradually our customers seem to have got used to a less sexy shoe; they are getting more experimental, wearing chunky sandals and jelly shoes with ankle socks. They are not styling the shoes with ugly clothes but experimenting with footwear. We like to say that they are dressing from the feet up."