Zayn - dating Little Mix singer Perrie Edwards for more than a year - romped with Courtney Webb hours after meeting her at a party in his £2.2million mansion.
Blonde Courtney, known to friends as CJ, said Zayn first assured her he was single. The truth only dawned after she spotted Perrie's belongings including jewellery, shoes, fake eye-lashes and make-up in his bedroom.
CJ, 21, told The Sun: "He's an absolute s***. How dare he - not only to me but to Perrie?
"I'm not a One Direction fan. I don't follow what they are up to. So when it clicked that he had a girlfriend I got more and more angry.
"I definitely don't think it's the first time he's done this. I feel sorry for Perrie - she doesn't know this is going on. It's wrong and it has to stop."
CJ met Live While We're Young singer Zayn in the early hours of Wednesday. She had been at central London's Dstrkt nightclub when friend Leon Anderson invited her and two friends to an "after-party at my boys'".