Prince Michael of Kent was willing to use his royal status for personal profit, and provide access to Russian President Vladimir Putin's regime, a Sunday Times and Channel 4 report says.
The Queen's cousin was filmed at a meeting in which undercover reporters were told he could be hired to make representations to the Kremlin.
His friend, the Marquess of Reading, later described him as "Her Majesty's unofficial ambassador to Russia".
The prince, 78, has denied the claims.
And in a statement, his spokesperson said that the marquess had "made suggestions which Prince Michael would not have wanted, or been able, to fulfil" during the covertly-recorded meeting.
They added: "As is standard practice, Prince Michael's private secretary made it clear to the company's representatives during their conversations that nothing could proceed without the agreement of the British Embassy and the help of the Russo-British Chamber of Commerce, of which Prince Michael is patron."
The Sunday Times and Channel 4's Dispatches allege that the marquess described Prince Michael as being able to meet Mr Putin and make representations on behalf of a company set up by the reporters.
The marquess is said to have warned them that the prince's services were "confidential", adding: "We're talking relatively discreetly here because we wouldn't want the world to know that he is seeing Putin purely for business reasons."
He estimated the prince could charge clients in the region of £50,000 for a five-day trip to Russia, the report claimed.
The prince is also said to have appeared willing to give the fictitious company his royal endorsement in a recorded speech for a fee of $200,000 (£143,000), and to use his home at Kensington Palace as a backdrop.
Prince Michael's office denied to the Sunday Times that he had a "special relationship" with the Russian president and said in a statement that he had not been in contact with Mr Putin or his office for almost 18 years.
The newspaper said the marquess later sent an email to the reporters which it described as suggesting he might have regretted being too candid about the prince and Russia.
The email said any arrangement would be subject to the prince's advisers to make sure everything was correct.
In a statement reported by the newspaper, the marquess said he had made a mistake and "over-promised" during the meeting with the undercover reporters and for that he was "truly regretful".
The Prince and Princess Michael of Kent are not working members of the Royal Family but have represented the Queen in the past. Before the pandemic, they attended around 200 engagements a year, according to the Royal Family website.
The website includes a biography which describes the prince as being "connected to Russia through his maternal grandmother" and as having "a strong interest in the country".
The biography said he "became the first member of the Royal Family to learn Russian, ultimately qualifying as a Russian interpreter".
The prince and princess do not receive public money.
He earns a living through a long-established consultancy company and the couple pay market rent and fees for their home at Kensington Palace, a spokesperson for the prince said.
His father, Prince George, was the fourth son of George V and the brother of George VI, the Queen's father.
Royals for Hire: Dispatches airs at 19:30 BST, Monday 10 May on Channel 4