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LONDON -- A woman's exposed bottom is Watch Four Riders (Hellfighters of the East) Onlineprobably the last thing you'd expect to see in a car park.

But that's nonetheless what Twitter user Alice Gray spotted in her gym car park in Cardiff, Wales.

UFit gym's advert features an image of a woman's bum alongside the text "there's better things to be stuck behind than the car in front". The ad has prompted a massive social media backlash after people accused it of sexism and objectification.

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Gray wasn't the only one to spot the advert and deem it sexist.

Twitter user @nattylou spotted it along the Cardiff half-marathon route, and complained to the marathon organisers.

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People responded to her complaint, stating the ad was "inappropriate".

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More Cardiff residents have taken to Twitter to call out the ad.

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And others feels the ad may make women less likely to join the gym...

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UFit gym did not immediately respond to Mashable's request for comment.