To whom it may concern,
I am writing this letter to apologize to you for the mistake i made in the work.I have acknowledged that my fault has brought you great trouble and unconviniences.I hereby express my deep sorrow for this matter and we are going to do our best to make it up to you.I assure you that this mistake will not happen once again and would appreciate very much if you could give us a chance to show our sincerity on this matter.
Best regards