A former Polish war refugee who met Prince Charles in Canada has said he likened some Nazi actions in Europe to those of Russia's Vladimir Putin.
Marienne Ferguson was speaking after meeting Prince Charles at a Nova Scotia immigration museum where she works.
As they discussed Hitler's takeover of countries, the prince "said something to the effect of 'it's not unlike... what Putin is doing,'" she recalled.
Clarence House said it would not comment on a private conversation.
The prince and the Russian president are both due to attend a D-Day anniversary event in France next month.
Speaking from China, President Putin's press secretary Dmitry Peskov told the BBC he was "not commenting right now" on the remarks.
By Peter Hunt, BBC Royal correspondent
It was, Marienne Ferguson told the BBC, "just a little remark. I didn't think it was going to make such a big uproar".
It has had significant ramifications because it was uttered by a man who will one day be head of state.
The British monarchy is in a period of subtle transition. Prince Charles is, slowly, taking on more of his 88-year-old mother's workload. As a result, there's a renewed focus on the type of king he will become.
Privately, Charles - a passionate prince - probably won't be too bothered that he's highlighted the actions of President Putin.
Others at Buckingham Palace and the UK Foreign Office are likely to be less sanguine.
And his critics - he has plenty for a person who'll one day occupy a unifying role - will see this as yet another example of an unelected hereditary heir meddling when he shouldn't.
On Monday, Prince Charles and the Duchess of Cornwall visited Pier 21, Canada's national immigration museum in Halifax, during their four-day tour of the country.
The prince chatted to museum volunteer Ms Ferguson, who left Poland for Canada just prior to the Nazi occupation.
The 78-year-old managed to escape to Canada along with her parents and two sisters, but other members of her family were sent to Nazi camps, the Daily Mail reported.
"He [Prince Charles] asked when I came to Canada, I told him 1939," Ms Ferguson said.
They had discussed how "Hitler was going into different countries and taking them over".
She added that she could not "exactly remember" the phrase the prince had used, but that he then "said something to the effect of 'it is not unlike what is now happening in Russia, what Putin is doing'".
"But it was only a moment... It was a very short remark."
The Russian president has been widely condemned since he annexed Crimea from Ukraine in March.
Former US Secretary of State Hillary Clinton made the same comparison between Mr Putin and Hitler in March, when Russia began issuing passports to Ukrainian citizens.
"Now if this sounds familiar, it's what Hitler did back in the '30s," she said.
Prince Charles has previously faced criticism at Westminster over his so-called "black spider" memos expressing his views on controversial issues to ministers.
Prime Minister David Cameron told BBC Radio 4's World at One he was "not going to comment on the private conversation of anyone".
Deputy Prime Minister Nick Clegg defended the prince over his comments, saying it was "clearly a private conversation".
He told BBC Breakfast: "I have never been of this view that if you are a member of the Royal Family somehow you have to enter into some Trappist vow of silence."
And UKIP leader Nigel Farage said that while he supported the monarchy there were times when "it might be better" for Prince Charles not to get involved in such things.
But Labour MP Mike Gapes, former Foreign Affairs Select Committee chair, said the prince should end his "freelance foreign policy".
Mr Gapes wrote on Twitter: "In constitutional monarchy policy and diplomacy should be conducted by parliament and government. Monarchy should be seen and not heard."
Commenting on whether the matter would be picked up in Russia, Sir Tony Brenton, a former British ambassador to Moscow, said the prince's latest comments were unlikely to have a seriously "damaging effect".
Clarence House said there were no plans for the prince to meet President Putin face-to-face.
Royal biographer Robert Hardman told BBC Radio 4's Today programme such incidents would be an "occupational hazard" for the prince as he takes on more of the "heavy lifting" on foreign tours from the Queen.
Andrew Child, a spokesman for the Republic campaign group, said the prince was developing "something of a track record for interfering in policy".