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Hillary Clinton has a “Jekyll and Hyde” personality that left White House staffers scared stiff of her explosive — and even physical — outbursts, an ex-Secret Service officer claims in a scathing new tell-all.

Gary Byrne, who was posted outside the Oval Office when Bill Clinton was president, portrays Hillary as too “erratic, uncontrollable and occasionally violent” to become leader of the free world, according to advance promotional materials exclusively obtained by Page Six.

The allegations from Byrne, a 29-year veteran of the military and federal law enforcement, threaten to derail her campaign days before she is expected to clinch the Democratic presidential nomination.

He describes Hillary Clinton as acting friendly one moment, then raging the next.

“What I saw in the 1990s sickened me,” he writes in the intro of the book, “Crisis of Character: A White House Secret Service Officer Discloses his Firsthand Experience with Hillary, Bill, and How They Operate.”

The book claims she repeatedly screamed obscenities at her husband, Secret Service personnel and White House staffers — all of whom lived in terror of her next tirade.

Secret Service agents had discussions about the possibility that they would have to protect Bill from his wife’s physical attacks, Byrne writes, and the couple had one “violent encounter” the morning of a key presidential address to the nation.

Meanwhile, a paranoid Hillary Clinton tried to have the Secret Service banned from the White House and once tried to ditch her security detail, Byrne says.

“Hillary Clinton is now poised to become the Democratic nominee for president of the United States, but she simply lacks the integrity and temperament to serve in the office,” he writes.

“From the bottom of my soul I know this to be true. And with Hillary’s latest rise, I realize that her own leadership style — volcanic, impulsive, enabled by sycophants, and disdainful of the rules set for everyone else — hasn’t changed a bit.”

The book isn’t set for release until June 28, but pre-orders have sent it to No. 1 on Amazon’s best-seller list.

Byrne — who was subpoenaed to testify before the grand jury that investigated Bill Clinton’s affair with intern Monica Lewinsky — claims in the book that he interrupted the president’s sexual shenanigans in the White House.

Byrne says he walked into a room where the president was “involved inappropriately with a woman” who was neither his wife nor Lewinsky.

And he says he once threw out a White House towel stained with a woman’s lipstick — and the president’s “bodily fluids.”

Byrne describes arriving for work one day in 1995 following a loud fight between the Clintons the night before.

The dust-up, he says, left a light blue vase “smashed to bits” and Bill sporting a “real, live, put-a-steak-on-it black eye.”

Byrne’s book amplifies earlier allegations of mistreatment of Secret Service personnel by Hillary Clinton — including in 2014’s “First Family Detail” by former Washington Post reporter Ronald Kessler and last year’s “The Residence: Inside the Private World of the White House,” which also detailed a fight between Bill and Hillary over the Lewinsky affair.

Hillary Clinton’s campaign downplayed Byrne’s book and compared him to Edward Klein, author of last year’s “Unlikeable: The Problem with Hillary.”

“Gary Byrne joins the ranks of Ed Klein and other ‘authors’ in this latest in a long line of books attempting to cash in on the election cycle with their nonsense,” spokesman Nick Merrill said. “It should be put in the fantasy section of the book store.”

A spokeswoman for presumptive GOP presidential nominee Donald Trump suggested Byrne’s book would be grist for his campaign as prepares to face Clinton in the general election.

“The issue of temperament is more a problem for Hillary Clinton,” said the spokeswoman, Katrina Pierson.

“I don’t recall Mr. Trump ever screaming at the Secret Service, calling them pigs, throwing vases across the room and clawing the face of his spouse, which Hillary Clinton has been reported to do,” she told CNN.

Additional reporting by Marisa Schultz and Bruce Golding