The Paradise Papers show the Duchy of Cornwall in 2007 secretly bought shares worth $113,500 in a Bermuda company that would benefit from a rule change.
The prince was a friend of a director of Sustainable Forestry Management Ltd.
The Duchy of Cornwall says he has no direct involvement in its investments.
A Clarence House spokesman said the Prince of Wales had “certainly never chosen to speak out on a topic simply because of a company that it may have invested in.
“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.
“Carbon markets are just one example that the prince has championed since the 1990s and which he continues to promote today.”
He added Prince Charles was “free to offer thoughts and suggestions on a wide range of topics” and “cares deeply” about the issue of climate change but “it is for others to decide whether to take the advice”.
Sir Alistair Graham, former chairman of the Committee on Standards in Public Life, said Prince Charles’s actions amounted to a serious conflict of interest.
He said: “There’s a conflict of interest between his own investments of the Duchy of Cornwall and what he’s trying to achieve publicly.
“And I think it’s unfortunate that somebody of his importance, of his influence, becomes involved in such a serious conflict.”
The leaked documents held by law firm Appleby show the Duchy of Cornwall also made offshore investments totalling $3.9m in four funds in the Cayman Islands in 2007. This is legal and there is no suggestion of tax avoidance.
A Duchy of Cornwall spokesman said Prince Charles voluntarily pays income tax on any revenue from his estate.
He added the estate’s investments “do not derive any tax advantage whatsoever based on their location or any other aspect of their structure and there is no loss of revenue to HMRC as a result”. (Courtesy BBC)