The Prince of Wales should register all his interests to prevent secrecy surrounding his private estate, a royal financial expert has warned.
Charles is embroiled in the Paradise Papers leak after it was reported that he lobbied for a change to two climate change deals after the Duchy of Cornwall invested in an offshore carbon credit trading company.
David McClure, the author of Royal Legacy - an investigation into royal finances - said the same rules should apply to the British Prince as to MPs, who are required to declare their financial interests.
"When Prince Charles makes a variety of speeches on ecological issues, while at the same time he has had a shareholding in companies, we know nothing about it," Mr McClure told the BBC’s Victoria Derbyshire programme.
"Maybe it is totally innocuous, the whole thing that has happened, but it would be far better if (the Prince) just registered all his interests. What is he hiding?"
The Duchy of Cornwall - which provides Charles with his annual income - paid 113,500 dollars (£58,000) in 2007 for 50 shares in Bermuda-registered Sustainable Forestry Management (SFM).
The heir to the throne later lobbied for a change to two climate change deals that would have directly benefited the business, the BBC and the Guardian reported.
A spokesman for Clarence House denied that Charles had spoken out on the two deals to benefit financially.
"The Prince has never chosen to speak out on a topic simply because of a company that the Duchy may have invested in," he said.
"In the case of climate change his views are well-known, indeed he has been warning of the threat of global warming to our environment for over 30 years."
The spokesman added that carbon markets were just one of many strategies Charles had championed to try to slow the pace of climate change.
Sir Alistair Graham, former chairman of the Committee on Standards in Public Life, said the British Royal Family should check their financial interests before delivering speeches if a change in public policy is being sought.
"Increased transparency would be a significant step forward. Individuals have to take responsibility for their own standards in these matters," he told the BBC.
"We expect members of the Royal Family to have the highest possible standards and I’m sure on most occasions they do.
"When there was a change of public policy being sought - it wasn’t achieved, thank goodness - which would have benefited this particular investment offshore, then we should expect Prince Charles or any other member of the Royal Family to declare their interests, and if they don’t know about their interests, that they check before they make public statements."
Margaret Hodge MP, a former chairwoman of the Commons Public Accounts Committee, told the Guardian: "What is clear is that there should be proper transparency of all investments made by the Duchy of Cornwall, that the Prince of Wales should not be involved in investment decisions, and that the Treasury should monitor the investments to ensure that the reputation and integrity of our Royal Family is protected."
Charles’s eldest son the Duke of Cambridge met two senior figures from the Duchy of Cornwall’s management team - secretary Alastair Martin and finance director Keith Willis - on Tuesday.
Kensington Palace described it as a regular scheduled meeting that has been in the diary for some time.
William is due to inherit the estate on his father’s succession to the throne, and will take the title of Duke of Cornwall.
The Duchy is worth more than £1 billion and provides Charles with an income of more than £20 million a year. It owns 53,000 hectares of land in 23 countries.
Charles is overseas in India on the last leg of his autumn tour. His talks with India’s prime minister Narendra Modi are likely to be overshadowed by the latest allegations to emerge from the Paradise Papers.
According to leaked documents, SFM, which counted one of Charles’s closest friends as a director, traded in credits from tropical and subtropical forests, but these type of credits were largely excluded from the EU’s Emissions Trading Scheme (EU ETS) and the Kyoto Protocol.
Four months after the share purchase, details of the company’s efforts to lobby for amendments to the two treaties were sent to the Prince, minutes from a board meeting suggested.
The documents were apparently sent to Charles at the request of millionaire banker and conservationist Hugh van Cutsem, whom the Prince met during his time at Cambridge.
Charles criticised the EU ETS and the Kyoto Protocol in speeches he gave shortly after receiving the papers, the reports said.
Neither the EU ETS or the Kyoto Protocol were changed as a result of the Prince’s lobbying, the BBC said.
The Duchy of Cornwall sold its shares in SFM in 2008 for 325,000 dollars (£246,000), according to the leaked papers, almost tripling its investment.
The papers also showed the Duchy of Cornwall made offshore investments totalling £3.9 million in four funds in the Cayman Islands in 2007, the BBC said.
This is legal and there is no suggestion the Prince received any tax benefit as a result of the investments in SFM or the funds in the Cayman Islands.
A spokesman for the Duchy of Cornwall said: "The Prince of Wales does not have any direct involvement in the investment decisions taken by the Duchy.
"These are the responsibility of the Duchy itself. The Duchy of Cornwall’s accounts are independently audited and presented to Parliament every year. The Prince voluntarily pays income tax on any revenue from the Duchy.