Heidi Montag allegedly made a lesbian sex tape with one of Hugh Hefner's former girlfriends.

The former 'Hills' star's estranged husband Spencer Pratt claims to have X-rated footage of the blonde beauty and her friend Karissa Shannon, 20, which he discovered in the attic of the Malibu home he previously shared with his spouse.

Karissa - who, along with her twin sister Kristina, dated Playboy founder Hugh for over a year until January 2010 - has admitted a tape exists, but doesn't believe the 26-year-old reality TV star has a copy but says she will launch legal action if the footage is made public.

According to gossip website TMZ, Spencer has told Heidi to cancel divorce proceedings and agree to do a reality TV show with him or he will sell the raunchy material - including two tapes of the former couple together - to adult film company Vivid Entertainment.

He said: "You cannot turn off the Speidi machine. I am a wild sexual freak and I love sex."

Spencer - who is believed to have tapes of Heidi after she had extensive cosmetic surgery and before the procedures -appeared to allude to the tape in a series of messages on twitter.

He posted: "Promises you the Citizen Kane of sex tapes. Honestly, Orson Wells makes a guest appearance.

"Recommends you wait for the Special Edition Sex Tape. Blu-Ray, director's commentary, & bloopers!

"The sex tape will not feature any bisexuality. It will however feature trisexuality. Also - a triceratops. (sic)"

His online comments provoked a furious response from both Heidi, 23, and Karissa.

Heidi - who filed for divorce in July - posted a direct message to her estranged spouse, which read: "F U!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! (sic)," while Karissa responded with: "F**K YOU!!!! (sic)"