Sunderland boss Roy Keane is facing an improper conduct charge by the FA following comments to Martin Atkinson during Saturday's defeat by Chelsea at Stamford Bridge. But top Premiership referee Howard Webb told BBC Radio Newcastle: "Passion is a big part of football. It hurts when we get things wrong, but there is also a line which can't be crossed."

Webb, who referees Arsenal against Manchester United this weekend, said: "We do not want to cut passion out of the game. It is what sets it apart from other sports for me. All we want to ask is that people realise there is a line that cannot be crossed.

"There has to be that little switch in the head which comes on and tells you 'that's enough'. It is down to respecting each other. But it is not just respect when things are going well, there has to be respect when mistakes are made as well. Any mistake made by a referee is not intentional."

He revealed he gets upset when he makes a mistake during a game and sees the evidence on screen afterwards. "It hurts when that happens," he said. "I will go home after a game where I have made a mistake and I will not be very pleased with myself.

"I will regularly look back at my performances and work out ways in which I could have done things better. I'd hate to think people feel we drive away from games happy with ourselves when we have made a mistake.

"We are only human after all but we want to be as accurate as we can be, as often as we can be."