Nicole Richie was reportedly removed from Las Vegas' Hard Rock Hotel and Casino early on Sunday morning.
The 26-year-old beauty - who has a five-month-old daughter Harlow with fiance Joel Madden - was said to have fought with a fellow reveler while waiting in line for the valet outside the hotel following a night of rowdy partying with Joel and his twin brother Benji.
An onlooker said: "Nicole started fighting with this woman while they were both waiting for the valet to bring round their cars. She ended up being so rowdy that she was escorted off the property by security."
However, Nicole's representative denied the fight, insisting: "Nicole was in Las Vegas but there wasn't an altercation of any kind involving her."
Nicole - who has previously been in rehab for drug addiction - recently revealed her life has never been better since she met Joel and gave birth to Harlow.
She said: "I'm happier than I have ever been but I take nothing for granted. I have great friends, a great family, a great place to live and a beautiful baby. I've gone through a lot of hardship in my past but I really feel like I've come out on the other end."
"I'd really like to show how much I appreciate the fact that I'm still here, because I almost didn't get that chance."