The feud between First Lady Melania Trump and Hunter Biden escalated sharply Thursday after President Joe Biden’s son responded with profanity to demands he retract a salacious and false claim linking her to convicted sex trafficker Jeffrey Epstein.

During a YouTube interview last month with Andrew Callaghan, Hunter Biden alleged that Epstein — who died in 2019 while awaiting trial — was responsible for introducing Melania to her future husband, Donald J. Trump. The claim, based on gossip and speculation, drew swift condemnation from Melania’s attorney, Alejandro Brito, who blasted it as “extremely salacious” and “false.”

In a letter first reported by Fox News, Brito warned Hunter that unless he issued a public apology and retraction, he would face a $1 billion defamation lawsuit for causing the First Lady “overwhelming financial and reputational harm.”

Given the chance to apologize in a follow-up appearance on Callaghan’s program Thursday, Hunter lashed out: “F— that, that’s not going to happen.”

He then tried to justify his smear by pointing to an unverified excerpt from a book by Michael Wolff, a discredited Trump critic President Trump has described as a “third-rate reporter.” Wolff’s claim had been repeated by The Daily Beast — which later deleted its article and issued an apology after receiving a similar legal warning from Melania’s attorney.

“I only can go by what people are saying, I don’t know,” Hunter admitted. Still, he taunted, “If [President Trump and the First Lady] want to sit down for a deposition and clarify the nature of the relationship between Jeffrey Epstein … I’m more than happy to provide them the platform to be able to do it.”

Melania’s spokesman, Nick Clemens, told ABC News: “First Lady Melania Trump’s attorneys are actively ensuring immediate retractions and apologies by those who spread malicious, defamatory falsehoods. The true account of how the First Lady met President Trump is in her best-selling book, Melania.”

In her memoir, Melania — a former model — wrote that she met Donald Trump in 1998 at a Fashion Week party in New York City, introduced by a modeling agent.

Hunter Biden’s refusal to back down comes as he faces ongoing legal and reputational troubles. In the closing weeks of his father’s presidency, Joe Biden issued him a sweeping pardon, sparing him from sentencing in Delaware and California on two criminal convictions.

Since then, Hunter has filed — and repeatedly dropped — multiple lawsuits targeting individuals and media outlets over material from his infamous laptop. One pending case, a defamation suit against former Overstock.com CEO Patrick Byrne, is expected to go to trial this fall.

In June, he abandoned a lawsuit against Fox News after accusing the network of airing sexually explicit images of him during a six-part Fox Nation series titled The Trial of Hunter Biden. The network stood by its coverage, calling Hunter’s claims “meritless” and “nothing more than a politically motivated stunt.”

For now, Melania Trump’s legal team is preparing for what could become one of the most high-profile defamation battles in recent memory — one that could cost Hunter Biden far more than just his already tarnished reputation.