Is it illegal to sign on behalf of someone without permission

Sir/Mam, I have given PoA to my father. Our property was given for rent executed by my PoA. But now we are in the court for eviction. Discovered from records that Tenant staff in 2018 had contacted the Electricity department with a hand written letter and hand signed by a tenant staff. As a general rule, signing on someone else’s behalf is legal so long as he/she is authorized to sign for them. Here if the person who had signed on behalf of my father has not been legally authorized or given authority, in my belief signing as/for someone else is legally considered either fraudulent or forgery. So should we not consider this as illegal and as a criminal/civil offence? If yes, what are the charges that we can make and how can we legally proceed?