Elon Musk was secretly recorded giving that ultimatum to remote-working Twitter employees while discussing plans for a 40-hour work week during a recent staff meeting. As you may recall ... some employees were clocking 12-hour shifts, 7 days a week to meet Musk's aggressive demands following his $44 billion purchase of the company.

A clip from the fiery recording was released by ABC News Thursday night ... and Musk can clearly be heard laying down the law.

"Let me be crystal clear, if people do not return to the office when they are able to return to the office -- they cannot remain at the company. End of story," Musk pointedly says on the audio.

A Twitter worker pushes back ... "Even if people returned to the office. The offices are separate offices -- we won't be in person anyways."

But, Musk isn't having it ... "You can still maximize the amount of in-person activity."

He then compares Twitter to his other company, Tesla, to drive home his point, "Tesla is not one place either, but you know, it's basically if you can, if you can show up at an office and you do not show up at the office, resignation accepted!"

Musk goes on to say plenty of Tesla employees work remotely, but they are "exceptions" and "exceptional people." He ends with ... "I totally understand if that doesn't work for some people. But that's the new philosophy for Twitter."

The world's richest man recently cleaned house at Twitter, slashing half the workforce. The fired employees sued Musk for violating federal and California laws by not giving them proper notice before they were canned and offering them unfair severance packages.

Shannon Liss-Riordan, an attorney for the laid-off workers, told ABC News she will use the audio clip as evidence to show Musk is eliminating remote work to avoid paying employees without giving them proper notice -- a violation of the WARN Act.

Meanwhile, Musk also announced Twitter may fall into bankruptcy if more people don’t sign up for his $7.99 monthly subscription service to offset slumping ad sales.