UPDATE: Prince Charles's comparison of Vladimir Putin with Nazi leader Adolf Hitler triggered a diplomatic row on Thursday when Moscow scolded the heir to the British throne for what it said was an outrageous attempt to sully Russia's reputation over Ukraine.
Charles's remarks, described by a royal source as "well-intentioned" and not meant to be public, caused a stir in Britain because the royal family is not expected to voice political views in public.
"If these words were truly spoken, then without doubt, they do not reflect well on the future British monarch," a spokesman for the Russian Foreign Ministry told a news conference.
"We view the use of the Western press by members of the British royal family to spread the propaganda campaign against Russia on a pressing issue — that is, the situation in Ukraine — as unacceptable, outrageous and low."
Russian diplomats are seeking an official explanation over the remarks which are especially emotive for a country that lost more people than any other in World War II.
Prince Charles, the heir to the British throne, has likened Russian President Vladimir Putin to German Nazi leader Adolf Hitler over his stance on Ukraine, according to a British newspaper.
During a royal tour of Canada, the 65-year-old prince told a Jewish woman who fled from Poland during World War II that "Putin is doing just about the same as Hitler," the Daily Mail said on Wednesday.
Charles's remarks, described by a royal source as "well-intentioned" and not meant to be public, were leading news reports in Britain where the government has been a strong critic of Moscow over its public support for pro-Russian rebels in eastern Ukraine.
Several high-profile figures have made a link between Russian moves against Ukraine, including annexation of Crimea, and German aggression leading to World War II.
In March, former US Secretary of State Hillary Clinton had to clarify remarks suggesting Putin's justification for his incursion into Crimea to protect ethnic Russians was reminiscent of claims made by Hitler over foreign territories.
"Now if this sounds familiar, it's what Hitler did back in the '30s," she said, later explaining she had not meant to make a comparison but said lessons could be learned from history.
Germany's conservative finance minister Wolfgang Schaeuble also denied making similar remarks a few weeks later.
Russia dismisses such comparisons and says Russian-speakers in eastern Ukraine are under threat from a Kyiv government in the sway of militant Ukrainian nationalists.
The Daily Mail said the prince's comments were made during a conversation with Marienne Ferguson, 78, who lost relatives during the Holocaust, when they spoke at a Museum in Halifax, Nova Scotia, where she works as a volunteer.
"I had finished showing him the exhibit and talked with him about my own family background and how I came to Canada," she told the paper.
"The prince then said 'And now Putin is doing just about the same as Hitler. I was very surprised that he made the comment as I know they (Britain's royals) aren't meant to say these things but it was very heartfelt and honest."
A spokeswoman for Charles's office said they did not comment on his private conversations.
"We would like to stress that the Prince of Wales would not seek to make a public political statement during a private conversation," she said.
Traditionally, Britain's royal family does not voice political views in public with the head of state merely a constitutional figurehead. During her long reign, Queen Elizabeth has never aired any such personal sentiments.
However, Charles has often courted controversy by voicing strong views on the environment, architecture and social affairs. Last year it was revealed he had held 36 meetings with government ministers since Prime Minister David Cameron's Conservative-led coalition took power in 2010.
Cameron's office said they would not be commenting but Liberal Democrat Deputy Prime Minister Nick Clegg said Charles, who has also taken on a greater share of the monarch's official duties as the 88-year-old queen slowly scales back her workload, should not be criticized for private remarks.
(Editing by Ralph Boulton)