"It's now got red wine spilled down the front because the last time I wore it my friend went like this" – she juts out her elbows – "and now it's everywhere," the actress told the Daily Telegraph in an interview.
Keira recycled her wedding dress to attend a charity gala with her husband James Righton
"I didn't quite realise the extent of it until I got home. It's quite impressively splattered. But, hey, a good night is a good night and when a dress has had its time, maybe it's had its time.
"I'm pretty sure I can get it out but, if I can't, I'm happy with the memory of a good night and the story of how the dress got destroyed."
Chanel face and muse Keira wore the white strapless, knee-length dress to marry James Righton from the indie band The Klaxons. She teamed it with a bespoke Chanel jacket.
"Karl [Lagerfeld] very kindly designed me a jacket to go with it," she said. "I'd worn the dress lots. It was my something old. And I like not making a big deal about it. I just wanted to be in something I had had a great time in, something I would have a great time in again."
Asked if she would want to pass it down to a daughter, she said, "I don't know. I'm not sure I'm into that. Can you imagine, say, your daughter is a different shape or she doesn't like it, and all of a sudden you're giving her pressure of wearing your dress?"
Earlier this month, Keira attended the Chanel show at Paris Fashion Week
Keira is one of Karl Lagerfeld's muses and recently attended the Chanel show at Paris Fashion Week.
Talking about why she loves working for Chanel, Keira said, "I get to dress up as somebody different. When I'm on the red carpet, it's, 'Oooh, who am I going to be tonight?'."