The on-loan midfielder walked out of the team's hotel after being told by manager Steve Bruce that he would not be in his 16-man squad for the live television clash at Upton Park.
"After being told I was not to be a part of the West Ham game I made a decision I regret, a decision made on the spur of the moment and while I was feeling totally dejected," the former England international midfielder told the Birmingham Mail.
"It was the wrong decision and unacceptable. I would like to apologise unreservedly to the manager, coaching staff, team mates and fans for my actions.
"There is no doubt we are in a battle and it is important that we all stick together."
Bruce revealed after the 3-0 defeat against Alan Pardew's side that he plans to hold talks with Butt over his actions.