Keira Knightley enjoyed having a body double get naked with Alexander Skarsgard for new movie The Aftermath, because it allowed her to take a break and eat cake.

The Atonement star has sworn off performing nude scenes onscreen since becoming a mother to daughter Edie in 2015, and now she only signs on to steamy projects if filmmakers agree to hire a lookalike to stand in for risque shoots.

Keira, who previously went topless for The Edge Of Love and The Duchess, made full use of her body double for the new period drama, set in the ruins of Germany following World War Two, and she was thrilled at how seamless the process was.

"They are exactly as uncomfortable as you'd imagine them to be," she said of filming sex scenes, "so this time I got a body double! It was great...! It's my head on somebody else's body!"

Keira was given full control over who would fill in for her for the sexy romp with Skarsgard, and she watched the action from a distance, with a sweet treat in hand.

"They (filmmakers) were like, 'What kind of body do you want?' I go, 'Wow, I'll have that one!'" the star laughed on U.S. morning show Live with Kelly and Ryan. "She was very nice, and they (body double and Skarsgard) apparently had a lovely time, and I got to not take my clothes off and eat cake on the other side of the room!"

Having the ability to turn down the nude scene and still get to complete the job has left Keira wondering why she hadn't thought up the idea sooner: "I've only just discovered this!" she exclaimed. "I'm like, 'This is amazing!'"

The Aftermath, which was directed by James Kent, also stars Jason Clarke and opens in U.S. theatres this week (15Mar19).