The 29-year-old Love Actually actress, who has been friends with her Atonement co-star Benedict, 37, for years, told ELLE Magazine about how the he reacted when the Observor's Mark Kermode called Keira 'Ikea Knightley' to imply that her acting was wooden.
"When I saw him [Benedict] again, I said, "Did you punch a journalist?" and he was like, "I did"," Keira told the magazine. "Everybody needs a friend like that."
Benedict Cumberbatch and Keira Knightley
Mark himself relayed the story to the Metro newspaper in 2009.
"An actor very gently punched me on air for saying something about Keira Knightley," he said. "I called her Ikea Knightley as her acting was so wooden."
Following alternative versions of the story, Mark took to Twitter to set the record straight about the incident.
"Benedict Cumberbatch did not punch me in the face. He punched me on the arm. In jest. It's on YouTube. And in my book," he wrote, adding that her did not insult the actress to her face as he has never met her.
Film critic Mark Kermode
Keira and Benedict are currently promoting their latest silver screen venture, The Imitation Game.
The film, which hits screens in November, is a biopic World War II codebreaker Alan Turing, played by Benedict, who helped crack the enigma code. Keira plays fellow cryptanalyst Joan Clarke, to whom Alan was briefly engaged.