Indonesian labour law treats the warning letter as a procedural step toward termination of employment under Article 154A paragraph (1) letter k of the Job Creation Law, yet the substantive requirements for its validity remain undefined. The problem becomes acute when a worker receives a disciplinary warning letter after taking religious leave that the employer had already approved, on grounds that no written rule regulates. This study aims to determine the validity requirements of a warning letter, the legal position of an approved leave, and the legal consequences of a warning letter issued for such leave. It applies normative legal research with statute, conceptual, and case approaches, examining Law Number 13 of 2003, Law Number 6 of 2023, Government Regulation Number 35 of 2021, Law Number 2 of 2004, and four industrial relations decisions retrieved from the Supreme Court Decision Directory, analysed qualitatively and prescriptively. The findings identify five cumulative validity requirements: a written normative basis, a concrete act meeting that norm, clarity of content, timeliness, and compliance with the time and level regime. Leave approval creates a legitimate expectation, reinforced for religious leave by Article 80 of the Manpower Law. Within this doctrinal framework, a warning letter lacking any violated norm is substantively defective, should not count as a step toward termination, and may be challenged through the rights dispute mechanism. The study contributes an operational validity test for courts, mediators, and employers, and recommends that implementing regulations codify these requirements.