Blackmailed at work

Hi, I work as a safety officer with a construction company. I recently did a safety audit which condemned an aspect of the lead contractors safety onsite. This audit was brought to the attention of the lead contractors project manager by their safety officer who I must send all audits too. Earlier today when I was alone in an isolated area I was confronted by their project manager and another manager of the lead contractor to whom my company are sub-contracted too. They told me that their could be consequences for me for bringing up their safety in my report. Their PM said if I ever again condemn their safety in my audits or bring up any issues with their safety manager that he could compile a condemning report against the safety of the company I work with and send it to the directors of my company and I would be in big trouble and that he could even have our company put off site which he said would not end good for me. Our company has an excellent safety record on site but it would be my word against theirs if they sent such a report. If I report the threat made to me it could be my word against theirs and I could lose out too as they are upper management and our company will not want to lose work with them. What should I do?

you have to be honest this safety is for people,cant risk their lives,go to the cops they have threatened you.