Lindsay Lohan had sex with her new boyfriend Riley Giles in the stairwell after meeting in rehab, according to the snowboarder's jilted fiancée.

Bree Tierney claims the Mean Girls star has ruined her life by stealing Giles from her, while the pair were patients at the Cirque lodge rehab centre in Utah, where Lohan spent much of this year.

Lohan and Giles have since been spotted out and about in Los Angeles - and Tierney is furious with both of them.

She told American magazine Star, "Lindsay Lohan stole my fiancé and wrecked my life. I loved Riley and Lindsay stole him from me.

"I've stuck by Riley through the good and the bad. Even after a car accident in Las Vegas, when Riley was arrested and I was left at the scene.

" That was his third or fourth arrest. We knew then he had to get help.

"So he went into rehab, and I waited for him. We lived together for three years and planned on being together.

"But then he confirmed that he had sex with Lindsay in one of the stairwells at Cirque that was it, we were done."

And Tierney insists she won't be taking her ex back when he and Lohan inevitably split.

She adds, "There's no way they are going to last. He's just looking for publicity.

"He's got Lindsay to get attention for his so-called snowboarding career. He's using her, and she's using him - nice, huh?" Tierney says. - Wenn