My deepest apologies.
B2I am very sorry.
I'm terribly sorry.
B2I am very sorry.
I beg your pardon.
B2I apologize; I'm sorry.
Forgive me.
B2Please pardon me.
I didn't mean to.
B2It wasn't intentional.
It was unintentional.
B2I didn't do it on purpose.
I sincerely regret...
B2I am truly sorry for...
I'm so sorry for the trouble.
B2I apologize for causing inconvenience.
I take full responsibility.
B2The fault is entirely mine.
It was a mistake on my part.
B2I made a mistake.
Please forgive my mistake.
B2Please pardon my error.
I should have...
B2I made a mistake by not...
I'm to blame.
B2I am responsible for this.
That was my fault.
B2I made a mistake.
I'm really sorry about that.
B2I apologize sincerely.
I understand your frustration.
B2I comprehend your anger/annoyance.
There's no excuse for my behavior.
B2My actions were unacceptable.
I hope you can forgive me.
B2I wish you would pardon me.
I'll make it up to you.
B2I will compensate for my mistake.
I really messed up.
B2I made a serious mistake.
I'm very sorry for letting you down.
B2I apologize for disappointing you.
I deeply regret my actions.
B2I feel a strong sense of sorrow for what I did.
I sincerely apologize for the oversight.
C1A formal apology for a mistake.
Please accept my deepest regrets.
C1Expressing strong remorse.