Conniving Prince Andrew has been privately threatening to humiliate his royal relatives by airing out their dirty laundry in an embarrassing tell-all — unless his brother King Charles doesn’t restore his palace paycheck and privileges, has learned.

“People thought Prince Harry’s memoir was bad. But what Andrew can reveal about certain members of his family would blow the lid off!” spilled an insider.

According to a source, the disgraced Duke of York, 63, has “an ax to grind” against Charles — who booted his sibling from his cushy digs in Buckingham Palace — and is prepared to unleash sordid details about the monarch, such as his many Indiscretions during his doomed marriage to the late Princess Diana!

Sources said vengeful Andrew may even address long-standing rumors about the kings’ flings with other men — more than six years after Charles, 74, was spotted puckering up with a much younger guy!

“Andrew has access to information that could prove embarrassing to Charles and, indeed, the whole family,” said the source.

“He could uncover details about this father Prince Philip’s ties to Hitler’s Nazi regime, and he wouldn’t be unwilling to spill the darkest secrets held by his own mother, Queen Elizabeth!” The spy said Andrew resents Charles for acting high and mighty when he’s really a hypocrite for marrying Diana in 1981 and continuing to carry on with mistress Camilla Parker Bowles, who’d later become his wife and queen consort!

Before Elizabeth’s death last year, the queen had stripped her favorite son of his royal duties due to his reputation being tarnished by his friendship with Jeffrey Epstein.