Actor Christopher Eccleston teared up as he apologised on TV for bullying someone when he was younger.
The former Doctor Who star spoke of the "shame he carries to this day" because of his actions as he revealed he was also bullied as a child.
In an interview on Good Morning Britain, he said: "There was a period when I bullied, for a very short period, and I did stop it myself but it's a shame that I carry to this day.
"The pop psychology of it is 'I bullied because I was bullied'. I don't know that I accept that. I know that I feel a great deal of shame.
"It's been a great shame and I would like to apologise to him. I know his name but I don't know where he is... I felt powerless so I made somebody else feel powerless."
The 52-year-old is now an active supporter of the Stand Up To Bullying campaign.
"I was bullied at school from the age of five by a girl who was seven and it happened every playtime. Playtime became terror time for me," he told presenters Piers Morgan and Susanna Reid.
"She would trap me against the wall, she would make me do sums, maths, and it really collared my life.
"I didn't want to go to school, I certainly didn't want to go out at playtime but this was in 1969 on a council estate in Manchester and, as I've grown up and thought about that girl, she seemed not as cared for physically as some of the other children and my thought is, 'God knows what was going on at home' because there was definitely a sense that this girl was very, very troubled and that's what I take from it really."
The actor said he did stop himself from being a bully, adding: "I can remember feeling dislike for myself while I did it so it's a very complex thing."
A host of celebrities are backing the first national Stand Up To Bullying Day, which has been organised for July 5 by the Diana Award charity.
The Diana Award was founded in memory of Diana, Princess of Wales and her "belief that young people have the power to change the world for the better".