Alexander Lebedev hit billionaire property developer Sergei Polonsky during a talk show on a Moscow Tv channel in 2011 and he has been banned from leaving Russia ahead of his upcoming trial on hooliganism charges.
He faces up to five years behind bars if convicted, but a slew of stars have spoken out in a bid to have Lebedev spared jail, according to Britain's Mail on Sunday.
Knightley has written a character reference for Lebedev and warned his charity work will be derailed if he goes to prison.
She writes, "I respect Alexander's work in the U.K. and abroad."
Sir Elton adds, "I have known Alexander for 15 years. He is a decent, generous, kind and thoughtful person and a man of integrity."
Kevin Spacey, Hugh Grant and supermodel Natalia Vodianova have previously supported the campaign to keep Lebedev out of jail.