Prince Michael of Kent, cousin of the queen, has received £320,000 ($514,000) from oligarch Boris Berezovsky, a Russian exile living in London, The Sunday Times reported.
The paper said that during the six-year period from 2002-08, a fund controlled by Berezovsky funneled 56 payments ranging between £5,000 and £15,000 through offshore companies into a family business owned by the prince's secretary.
The transfers were revealed in documents submitted to the High Court ahead of an upcoming case.
Berezovsky himself told The Sunday Times that "there is nothing underhand or improper about the financial assistance I have given Prince Michael. It is a matter between friends."
Prince Michael, who has well-publicized financial problems, has received financial support from other sources, including the queen, who paid rent for his Kensington Palace home, which costs £100,000 per year. He has also faced allegations of using his title to attain free trips and supplement his income.
Unlike other members of the royal family, the prince does not earn money from public funds.
Prince Michael of Kent is the grandnephew of Tsar Nicholas II. He is fluent in Russian, regularly travels to Russia and is a patron of numerous charitable foundations that support Russian culture and education.