Hi Aanchal,
As has been correctly pointed out by our friends, a warning letter cannot be issued untill the charges of misconduct are proved against an employee in question.
However, a warning or a letter of cautioning may be issued to an employee, even before a domectic enquiry, who has committed an act of mosconduct, not so severe in nature, in order to give the employee a chance to rectify him/herself from such acts in future, if the Standing Orders/Service Rules by which the employee is governed provide as such.