Teasing details of the never-before-seen dress, the 34-year-old reportedly showed the Elle UK team a photograph.
"It has long, layered lace skirts and tiny pink flowers embroidered all over it," the story reads. "She wore it 'for the ceremony we had just for our friends, in a wood, because you have to have flowers on a dress in a wood.'"
So where is Knightley's secret second wedding dress now?
Like most brides, the Oscar-winning actress has yet to have the Valentino gown cleaned and it remains hanging in her closet.
"Could you frame it? I don't have the wall space. Then I thought I could cut it up and make it into something I could wear, but then that’s a shame," she told marie claire UK.
"It's not ballgown, but it had a bit of a train. Unless I just wear it again and ruin it, like a party somewhere. Or maybe somebody could organise a wedding dress party, like in Friends. Yeah, that's quite cool."
The silver screen star was widely applauded for recycling a tulle Chanel dress to wed the Klaxons bandmate.
When photographs of the bride-to-be decked in the frock emerged online, fans were quick to note that Knightley had previously worn the look on the red carpet.
Knightley reportedly first donned the Karl Lagerfeld number for the Finch & Partners' pre-Bafta event in 2008.
If that wasn't enough to inspire brides out there to wear their wedding day ensemble more than once, she also revisited the look after taking it down the aisle, at the SeriousFun gala in December, 2013.
But that may have been the last time she's spotted in the strapless frock, as it has yet to recover from a slight mishap on an evening out.
"It's now got red wine spilled down the front because the last time I wore it my friend went like this and now it's everywhere," Knightley told The Telegraph.
"I didn't quite realise the extent of it until I got home. It's quite impressively splattered," she continued. "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."
After tying the knot, Knightley and husband James Righton welcomed their first daughter, Edie, in 2015. The couple later went on to have a second daughter in 2019.