Melania Trump has threatened to sue Hunter Biden for more than $1bn (£736.5m) in damages if he does not retract comments linking her to Jeffrey Epstein.
Mr Biden, who is the son of former US president Joe Biden, alleged in an interview this month that sex trafficker Epstein introduced the first lady to President Donald Trump.
“Epstein introduced Melania to Trump. The connections are, like, so wide and deep,” he claimed.
Ms Trump’s lawyer labelled the comments false, defamatory and “extremely salacious” in a letter to Mr Biden.
Hunter Biden. File pic: AP
Her lawyer wrote that the first lady suffered “overwhelming financial and reputational harm” as the claims were widely discussed on social media and reported by media around the world.
The president and first lady previously said they were introduced by modelling agent Paolo Zampolli at a New York Fashion Week party in 1998.
Mr Biden attributed the claim that Epstein introduced the couple to author Michael Wolff, who was accused by Mr Trump of making up stories to sell books in June and was dubbed a “third-rate reporter” by the president.
The former president’s son doubled down on his remarks in a follow-up interview with the same YouTube outlet, Channel 5 with Andrew Callaghan, entitled “Hunter Biden Apology”.
Asked if he would apologise to the first lady, Mr Biden responded: “F*** that – that’s not going to happen.”
He added: “I don’t think these threats of lawsuits add up to anything other than designed distraction.”
Ms Trump’s threat to sue Mr Biden echoes a strategy employed by her husband, who has aggressively used legal action to go after critics.
Public figures like the Trumps must meet a high bar to succeed in a defamation suit like the one that could be brought by the first lady if she follows through with her threat.
In his initial interview, Mr Biden also hit out at “elites” and others in the Democratic Party, who he claims undermined his father before he dropped out of last year’s race for president.
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The letter threatening legal action against Mr Biden is dated 6 August and was first reported by Fox News Digital.
It was addressed to Abbe Lowell, a lawyer who has represented Mr Biden in his criminal cases. Mr Lowell has not yet commented on the letter.
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This comes as pressure on the White House to release the Epstein files has been mounting for weeks, after he made a complete U-turn on his administration’s promise to release more information publicly.
The US Justice Department, which confirmed in July that it would not be releasing the files, said a review of the Epstein case had found “no incriminating ‘client list'” and “no credible evidence” the jailed financier – who killed himself in prison in 2019 – had blackmailed famous men.
