No warning letter without due enquiry into what exactly did employee forge-purpose and how he used forged document etc.
On due enquiry and if found harmless or a minor breach then probably warning letter can be given and a circular to all employees about use of organisation letterheads, documents etc.
Hurried warning letter can become a problem because this punishment will stop you from any further serious action if something emerges later on due to his forgery.
so take steps with care and caution.
15th April 2018 From India, Pune
If standing orders act is applicable to your establishment , warning can be issued after receiving employee's reply to show cause notice.
16th April 2018 From India, Mumbai
17th April 2018