Hillary Clinton
Hillary ClintonReuters

Democratic presidential frontrunner - and by now presumptive nominee - Hillary Clinton could be facing another potentially damaging scandal, as a former Secret Service agent stationed at the White House prepares to release a book which he claims reveals damning evidence that she "lacks the integrity and temperament" to be president.

Gary Byrne, who guarded the door of the Oval Office during Bill Clinton's presidency, described Hillary as "volcanic, impulsive, and enabled by sycophants," and says what he saw of the then First Lady's behavior "sickened me."

He also claims to have "personally observed President Bill Clinton's infidelities." Indeed, it was complaints lodged by Byrne which resulted in Monica Lewinsky being transferred from the White House to the Pentagon.

Byrne describes how in one episode, which occurred in 1995, the first couple could be heard screaming at each other through the Oval Office door. He says he heard Hillary's "booming voice" particularly loudly, and recounts that the president turned up to work the next morning with a "put a steak on it" black eye.

President Clinton's secretary told Byrne the black eye was the result of an allergic reaction to coffee. 

He describes a paranoid first couple "so intent on destroying their real or imagined enemies that governing became an afterthought," and accused Hillary Clinton of having "disdain for the little people."

Denying his motives for releasing the book are political, he says he feels duty-bound to highlight how the Democratic frontrunner is unfit for office.

Despite that, the book is due to be released on June 28 - just days before the Democratic Convention.

Neither Clinton have commented on the book.