Keira, 29, made the confession during an appearance on The Jonathan Ross Show - appearing alongside GBBO judge Paul Hollywood - and revealed how she tried to convince her agent to let her leave early.
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Great British Bake Off fan Keira Knightley with judge Paul Hollywood
"There was a real moment where I was like, 'No, but no, you don't understand'," she said. "I did actually say to my agent, 'Is it alright, because I have seen the film twice before and I love the film, but can I just walk the red carpet and just go back and watch it?'"
Her request was turned down – but Keira did manage to leave the afterparty to watch it on catch-up shortly afterwards.
"I did actually leave the party a bit early so I could watch it on iPlayer afterwards so that nobody would tell me what happened," the star said. "I turned off the radio on the way back so I didn't know."
Keira Knightley also spoke of her dislike of social media during the interview
During the interview, Keira also spoke about her dislike of social media. "I did join Twitter for about 12 hours because I tried to be down with the kids and it just creeped me out," she said.
"I didn't post anything and I was under a false name and I think because Chloe (Moretz) followed me, suddenly all these people starting following me and posting 'I'm having a cup of tea now', and I just got completely freaked out!"