Jennifer Aniston threw a star-studded birthday bash for Justin Theroux on Saturday night (10.08.13).

The 'We're the Millers' star, 44, and her fiance were joined by 75 guests, including Ellen Degeneres, Ben Stiller, Jimmy Kimmel, Tobey Maguire, Laura Dern, John Krasinski, Don Cheadle and Jason Bateman, for the lavish party at their $21 million Bel Air mansion in Los Angeles.

Kristen Bell, Isla Fisher, Samantha Ronson, Will Arnett, Malin Ackerman and Dax Shepard were also present to help Justin celebrate his 42nd birthday.

A source told People.com that guests, who dressed down for the occasion despite rumours the couple was planning a surprise wedding ceremony, were treated to a drinks reception on the patio before going inside the newly renovated 8,500 sq. ft. property to toast Justin and enjoy a relaxed buffet, prepared by Jennifer's personal chefs.

The couple were initially thought to be preparing to tie the knot at the party, to mark the one year anniversary of their engagement, as they received a large delivery of Champagne on Friday (09.08.13), secured parking restrictions around their home, and Jennifer's personal hairstylist Chris McMillian was also spotted arriving to do her hair.

Jennifer, who was previously married to Brad Pitt, insisted last week that she was in no rush to wed.

She said: ''We just want to do it when it's perfect, and we're not rushed, and no one is rushing from a job or rushing to a job. And, you know, we already feel married.''