William Shatner and Carrie Fisher have waded into that long-standing battle over which is better, Star Wars or Star Trek, posting videos to YouTube in which they argue over special effects and who would best fit into their old, revealing costumes.

Shatner fired the first shot back in September, when he dismissed Star Wars as "derivative" of Star Trek and criticized its special effects.

He also said the girls who appeared on Star Trek were "prettier than the girls on Star Wars."

"Princess Leia, as beautiful as she was and as wonderful an actress as she is, can't compare to the marvelous heroines we had on Star Trek," Shatner tells a female interviewer in the video.

Shatner did concede that given the chance, Captain Kirk would "absolutely" run off with Princess Leia.

But earlier this month, Fisher upped the ante in a video of her own in which she suggests the two films "aren't in the same league," and challenges Shatner to a "costume-off."

"I want to see him in his clingy outfit again," she says, alleging Shatner once borrowed her infamous metal bikini.

Fisher also praised her "space buns" for being "so much better than (Leonard) Nimoy's ears."

Near the end of her video, Fisher says Shatner auditioned for the role of Darth Vader but, despite being in full costume, "he couldn't do it."

"If you see Bill Shatner, call him Han Solo," she says.

After Fisher's video hit YouTube, Shatner filmed a retort in which admits he "grovelled" to Star Wars producers for the role that eventually went to "this big heavy-voiced guy," and joked that he would have played the villain in a squeaky, Mickey Mouse-type voice.

But Shatner wields his phaser once again, saying Star Wars had "the best special effects" but "forgot about story and character and plot development." He then turns his weapon on Fisher.

"I don't know if we're ready for you in a bikini," he says to the camera. "My costume was made of stretch material, so there wouldn't be any difficulty giving it a little push here and there for me to go into my original wardrobe. But you in a bikini, now, needs a little more than push and pull. It needs a lot of uplift. See ya Carrie."

Sci-fi fans everywhere are anxiously awaiting Fisher's response. Meanwhile, here are the videos: