Belgian referee Frank De Bleeckere and his two assistants, Peter Hermans and Alex Verstraeten, failed to chalk off Berbatov's goals in Tuesday's Group E clash at Old Trafford. They also wrongly ruled out a Wayne Rooney strike for offside.

Verstraeten said: "I admit to the mistakes, but we also ask for understanding. Offside decisions are the hardest to judge. We are only human flesh and blood."

He added: "This is the first time that we have made a mistake in 52 international matches.

"It is our bad luck that the mistakes came at goals. It is no consolation that Manchester United would have won without these decisions.

"Some players argue their case in front of us, but no Celtic player protested.

"That proves that the offside decisions were very tight. At the end of the match, I was surprised when I saw the television images."

De Bleeckere added: "At the moment itself, we were not aware of the error. There was no protest. I walked off the field with a good feeling. But when you see the images, it's a shame.

"The officials are a team. We did okay at the European Championships and in the Champions League. We must help each other and see it as a learning curve."