A file note about verbal warnings can be recorded.
Whenever you give a staff member a "verbal warning" or even having a general discussion about performance issues, you should ALWAYS record the gist of the discussion AND any agreed actions, on the employee's file.
By keeping a written record of all such discussions, you can build your case if it becomes necessary to dismiss the employee.
Not sure what happens in India, but in many countries, if a dismissal case comes to court, an employer who has good evidence of all actions taken prior to dismissing the employee (e.g. verbal warnings, performance plans, written warnings etc), would have a very good chance of winning the case.