Lawyers representing a woman who has alleged Prince Andrew and law professor Alan Dershowitz had sex with her when she was a minor are suing Mr Dershowitz for defamation.
The lawyers accuse Mr Dershowitz of mounting a media assault on their reputation and character.
The development comes a day after he began legal proceedings to clear his name.
Both Buckingham Palace and Mr Dershowitz deny the claims.
On Monday, Mr Dershowitz filed papers at a court in Florida, where the case is being heard, to contest what he described as “absolutely outrageous claims”.
The former Harvard law professor has asked for his name to be removed from documents which accuse him and Prince Andrew of having sexual relations with Virginia Roberts, known in court as Jane Doe #3, who was under the age of consent in the US at the time of the alleged incidents.
In media interviews, Mr Dershowitz accused Ms Roberts’ lawyers, Paul Cassell and Bradley Edwards, of “unethical” behaviour warranting disbarment.
The pair said their reputations had been damaged by such comments and that they were seeking damages.
In response, Mr Dershowitz said he was “thrilled” to be sued as it would give him an opportunity to disprove the two.
The allegations against Prince Andrew and Alan Dershowitz emerged in evidence submitted to a US court in a case involving financier Jeffrey Epstein.
Two women, referred to as Jane Doe #1 and Jane Doe #2, are suing the US government. They argue that it failed to protect their rights when it entered a plea deal with Epstein, who spent time in jail in 2008-9 for a sex offence with a minor.
Two more – Virginia Roberts and a woman known as Jane Doe #4 – now want to join the case.
Documents filed in a Florida court last week allege that Ms Roberts was sexually trafficked by Epstein, who forced her to make herself available for sex to “politically connected and financially powerful people”.
Ms Roberts claims she was forced to have sex with the prince on three occasions – in London, New York and on a private Caribbean island owned by Epstein – between 1999 and 2002, when she was under age according to US law.
Epstein and Prince Andrew were formerly friends and the pair were photographed meeting in December 2010, after the tycoon had served his prison sentence.
The prince later had to apologise for his friendship and stepped down as the UK’s special representative for trade and investment.
Buckingham Palace has said the sex claims against the duke lack “any foundation”.
It is likely the 54-year-old’s first public appearance will be at a reception at the World Economic Forum in Switzerland later this month.
A royal aide has said the prince will “move on and push ahead” with his work, describing him as “resilient”.