Poppy Delevingne threw a wild retro-themed party at the U.K. capital's posh Groucho Club, which was attended by her supermodel sister and famous female friends including actresses Sienna Miller and Jaime Winstone and Tv presenter Alexa Chung.
Styles, who was once linked to Cara, did not want to miss out on the fun - he turned up to the bash, which was in celebration of Poppy's final days as a single woman ahead of her wedding to James Cook.
The One Direction star donned a blond wig to impersonate a member of 1990s band Hanson, along with radio star Nick Grimshaw and a female pal.
The trio partied at the club until the early hours of Sunday morning (01Dec13), when Styles left the venue wearing a huge elephant mask.
He later posted a snap of himself posing with Miller on his Instagram, and wrote: "I was part of Hanson."
Meanwhile, supermodel Cara had an embarrassing end to the night - she gave a woman driver a shock by hopping into the backseat of her car after mistaking it for a taxi cab.