Dear Bala, the aim of awarding punishment is to correct behavior. Your query is on issuing a warning letter directly. Yes, it can be issued but only for minor offenses.
However, before issuing a warning letter, be clear that you have all the available facts or even material evidence of the misconduct at hand. There should be no room for assumption.
The benefit of issuing a show cause notice is that the company gives the accused employee a complete chance to defend his/her position. If any issues are overlooked, the recipient of the show cause notice may bring them to light, which helps in determining the appropriate punishment.
Thanks,
Dinesh Divekar