"The Hills" dudes face off! Sort of...

JASON WAHLER is putting SPENCER PRATT up against the rails for claiming there is a LAUREN CONRAD sex tape.

"I do not have a sex tape of Lauren Conrad and one does not exist," her former flame told Usmagazine.com. "Spencer Pratt is lying again to get attention."

"Jason's statement couldn't be more transparent, Spencer shot back. "Notice how he uses present tense, 'I DO not have a sex tape... one DOES not exist.'"

Spencer added to the mag: "We all know that it existed, that he tried to sell it and is now covering up to make himself look better."

Both Spencer and girlfriend HEIDI MONTAG told TYRA BANKS on her talk show Tuesday that they believe a sex tape exists.

"I know for 100 percent fact it did exist -- 1000%!" Spencer proclaimed.

Heidi said the video was "not a rumor. From what I have heard from confirmed sources ... her ex-boyfriend was going around trying to sell it."

But when Tyra pressed on, asking if she'd actually seen the tape, Heidi seemed flustered.

"Well, I mean, I am not sure she has it anymore ... but from what I've heard..."

She then cryptically added, "I am not really allowed to elaborate."

The alleged video is a holy grail of sorts for "The Hills." It is the reason L.C. dumped Heidi as her BFF after she and Spencer supposedly spread rumors about her appearing in a sex tape with Jason.

"Lauren is my friend," Jason added in his statement, "and it is insulting to her to suggest this."