In an unusual move, FBI agents working the Hillary Clinton email case had to sign a special form reminding them not to blab about the probe to anyone unless called to testify.

Sources said they had never heard of the “Case Briefing Acknowledgment” form being used before, although all agents must initially sign nondisclosure agreements to obtain security clearance.

“This is very, very unusual. I’ve never signed one, never circulated one to others,” said one retired FBI chief.

An FBI agent currently on the job admitted, “I have never heard of such a form. Sounds strange.”

Meanwhile, FBI agents expressed their “disappointment” over FBI Director James Comey’s decision not to recommend charges against Clinton, sources close to the matter told The Post.

“FBI agents believe there was an inside deal put in place after the Loretta Lynch/Bill Clinton tarmac meeting,” said one source.

Another source from the Justice Department was “furious” with Comey, saying he’s “managed to piss off right and left.”