BBC NEWS ONLINE: Can you be sustainable if you're into fast fashion online?

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Patsy Perry, a fashion marketing lecturer at the University of Manchester, said the way these websites are designed encourage people to buy more.

"Some of them might have new products every week," she told Radio 1 Newsbeat. "They really trade on very very frequently renewing collections, more so than many traditional high street retailers."

As well as the availability of new clothes, the trend of posting outfits on social media makes some women feel like they can't wear the same thing more than once, says Patsy.

"We see celebrities doing that - all the big influencers you barely see them wearing the same thing twice," she adds.

Changing your shopping habits is difficult, says Patsy, from the University of Manchester: "It's like fast food, it's easy, you get a quick fix, a buzz when you buy it."

But the main driver of change would be an "initiative where people don't feel ashamed of wearing the same thing again," she added.