Manchester United boss Sir Alex Ferguson admits he has put pressure on referees with his criticism in the past.

"I still believe we should be doing more to protect our creative players from reckless tackling, but there are some factors that make life very difficult for referees," said Ferguson, in his programme notes for yesterday's victory over Aston Villa.

"The habit of surrounding a referee in an attempt to influence a decision, be it in defence of one of their players or to try and get an opponent punished, has got out of hand. I see this as a club responsibility because it is very difficult for a referee to deal with when he is faced by dissent in such big numbers.

"We had a pivotal moment ourselves when our players chased Andy D'Urso across the pitch (against Middlesbrough in 1999/00). I was incensed and let the players know it must not happen again and it hasn't.

"While I have questioned the ability of some referees, I have never doubted their integrity. Yes, I have contributed to putting them under pressure, but that's for the protection of my players. Graham Poll wrote critically of me recently but he can be assured that while there will continue to be criticisms of officials' decision-making, I don't think there is a manager in the game who would question their integrity. Our game is thankfully free of that kind of corruption."