It is a cardinal rule in law that due process must always be observed.  In labor cases, employers have often lost cases because they have not complied with procedural due process in the dismissal of their employees.  In the case of San Antonio vs NLRC, et al.[G.R. No. 100829.  November 28, 1995.], the Supreme Court held that:

"The rudiments of due process cannot be lightly ignored. Proper compliance with the twin requirements of notice and hearing are conditions sine qua non before a dismissal may be validly effected.  Elucidating, the Court, in Pepsi-Cola Bottling Co. v. NLRC, (210 SCRA 277, 286) explained: "The law requires that the employer must furnish the worker sought to be dismissed with two (2) written notices before termination of employment can be legally effected: (1) notice which apprises the employee of the particular acts or omissions for which his dismissal is sought; and (2) the subsequent notice which informs the employee of the employer's decision to dismiss him (Sec. 13, BP 130; Sec. 2-6 Rule XIV, Book V, Rules and Regulations Implementing the Labor Code as amended). Failure to comply with the requirements taints the dismissal with illegality. This procedure is mandatory; in the absence of which, any judgment reached by management is void and inexistent."  The second notice must be given the employee after due hearing.   The hearing requirement is not to be considered a mere technicality but one of substance to which every employee is entitled in order to at all times assure that the employer's prerogative to dismiss or lay-off is not abused or exercised in an arbitrary manner.  Consultations and conferences may not be valid substitutes for actual observance of notice and hearing.  Any procedural shortcut, that effectively allows an employer to assume the roles of both accuser and judge at the same time, should not be countenanced.  Not excluded from the rule are confidential and managerial employees; they themselves cannot be arbitrarily dismissed without such just causes as must be reasonably established in appropriate investigations.  Shortly after petitioner, in compliance with the company's directive, had explained why he should not be disciplinary dealt, he received forthwith the company's "decision" dismissing him from employment. No hearing, or a semblance thereof, was conducted apparently because the company believed that the case was res ipsa loquitur in character."

The Supreme Court further elucidated the twin notice requirement in the case of Nitto Enterprises vs NLRC [G.R. No. L-114337.  September 29, 1995.], to wit:

"There is an abundance of cases wherein the Court ruled that the twin requirements of due process, substantive and procedural, must be complied with, before valid dismissal exists.  Without which, the dismissal becomes void.The twin requirements of notice and hearing constitute the essential elements of due process. This simply means that the employer shall afford the worker ample opportunity to be heard and to defend himself with the assistance of his representative, if he so desires. Ample opportunity connotes every kind of assistance that management must accord the employee to enable him to prepare adequately for his defense including legal representation 

As held in the case of Pepsi-Cola Bottling Co., Inc. v. NLRC: 

"The law requires that the employer must furnish the worker sought to be dismissed with two (2) written notices before termination of employee can be legally effected: (1) notice which apprises the employee of the particular acts or omissions for which his dismissal is sought, and (2) the subsequent notice which informs the employee of the employer's decision to dismiss him (Sec. 13, BP130, Sec. 2-6 Rule XIV, Book V, Rules and Regulations Implementing the Labor Code as amended). Failure to comply with the requirements taints the dismissal with illegality. This procedure is mandatory; in the absence of which, any judgment reached by management is void and inexistent (Tingson, Jr. vs. NLRC, 185 SCRA 498 [1990]; National Service Corp. vs. NLRC, 168 SCRA 122, Ruffy vs. NLRC. 182 SCRA 365 L [1990])."