The director who worked with Knightley on the 2013 romantic comedy Begin Again said she was a "supermodel" who didn't get her character and brought an entourage with her everywhere she went.
"I'm ashamed of myself that I could say such things and I've been trying to account for what they say about me," Carney tweeted. "In trying to pick holes in my work, I ended up blaming someone else."
He posted the apology with the caption "From a director who feels like a complete idiot."
Carney concluded, "Keira was nothing but professional and dedicated during that film and she contributed hugely to its success," and added his "arrogant and disrespectful" comments were "not something I could ever justify, and will never repeat."
Carney made his criticisms while promoting his latest film, Sing Street. Since then, former colleagues have jumped to Knightley's defense.
"She was an absolute dream," Joy Gorman said. Gorman produced Knightley in Seeking a Friend for the End of the World. "Professional, prepared, kind and lovely."
Mark Romanek, who directed Knightley's 2010 film Never Let Me Go, tweeted that Carney was an "#arrogants---head" while saying the actress was "utterly spectacular on every level."
From a director who feels like a complete idiot. pic.twitter.com/vfO8m4U2Hl— John Carney (@jayceefactory) June 1, 2016