Prince Andrew is facing accusations that he exploited his role as trade envoy for the UK to help his multi-millionaire friend.
Fresh allegations have emerged as the Duke of York continues to deal with the fall-out from his "car crash" interview about his relationship with disgraced financier Jeffrey Epstein.
The Mail on Sunday reports that email suggest Andrew "exploited his taxpayer-funded role to work behind the scenes" for financier David Rowland, who the paper refers to as his "close friend".
The paper reports that the emails suggest the duke was, while on official trade missions, quietly plugging a private Luxembourg-based bank for the super-rich, owned by the Rowland family.
Buckingham Palace has denied the claims, insisting that Andrew was promoting British interests and "not the interests of individuals" during his 10-year sting as the UK’s special representative for international trade and investment.
A statement said: "The Duke of York was the UK’s Special Representative for International Trade and Investment between 2001 and July 2011 and in that time the aim, and that of his office, was to promote Britain and British interests overseas not the interests of individuals.”
Former Foreign Affairs Committee member Chris Bryant, who was in office when the duke held the role, told the newspaper that his former committee or the Public Accounts Committee should launch an inquiry.
The Mail on Sunday's investigation claims the Prince allowed the Rowlands to "shoehorn meetings into his official trade hours" and "passed them private government documents they have no right to see".
It also claims Andrew co-owned a business at the time with the family in the Caribbean.
The fresh allegations come after Andrew stepped down from public duties in the wake of his disastrous Newsnight interview about his association with Epstein, who took his own life in jail earlier this year while facing sex trafficking charges.
During the interview, Andrew said his relationship with Epstein gave him the opportunity to meet people and help prepare for the official role.
The duke stepped aside as trade envoy in 2011 after it became public he had stayed with Epstein following the convicted sex offender’s release from jail.
Virginia Giuffre, one of 16 women who say they were abused by Epstein, claims she was forced into a sexual encounter with Andrew – an allegation he strenuously denies.
In another blow for the duke, Ms Giuffre’s first UK television interview is to be broadcast on Monday.
A Panaroma programme entitled The Prince And The Epstein Scandal will see Ms Giuffre tell her story and reveal new details about her time with Epstein, the BBC said.
Ms Giuffre claims the duke slept with her on three separate occasions, twice while she was underage.
The palace branded the allegations “false and without any foundation”, stating “any suggestion of impropriety with underage minors” by the duke was “categorically untrue”.
The PA news agency was unable to contact Mr Rowland. The Mail on Sunday said he declined to comment for legal reasons.