Manchester City have lodged a complaint with Sky Sports against Manchester United legend Roy Keane.

The Daily Mail says former United captain Keane stuck the boot in after Kyle Walker gave away a penalty for a foul on Sadio Mane in a 1-1 draw with Liverpool earlier this month.

When asked how Mane earned the penalty, Keane said: "Because he's up against an idiot", before later adding "he's a car crash, keeps making these types of mistakes and is rightly punished for it".

City have no issue with a pundit questioning a player's actions or decisions on the pitch but they believe by calling Walker "an idiot", Keane stepped over the line.

Sky declined to comment, but the broadcaster does say its pundits are there to express individual opinions.