An undercover TV report will reveal serious incompetencies and systemic failures within the Royal Mail when it goes to air on Monday.
Reporters for Channel 4's Dispatches will find an "antiquated system with lax security, poorly trained agency workers ... damaged and defective equipment and allegations of stealing".
The journalists joined the company just before the busy Christmas period, shortly after an industrial truce was found.
While there, they found that disputes over working times and the size of rounds were still disrupting service.