1.Lodge a complaint with the consumer forum for agaisnt this deficient act of the bank. The bank can not accept your authorisation letter so casually so as to issue bank statements.
2. Since this a forgery you must lodge a complaint with local police station for this forgery and criminal personation.if you suspect someone you can mention his name as well in the complaint.On this basis the Police is bound to register FIR and arrest the culprit.
3. Do not worry as if bank honours a cheque nering your forged signature then it is bound to make good the loss caused to you.