In a withering verdict on the actions of the Russian president in Ukraine, he told a woman who lost relatives in the Nazi Holocaust: ‘And now Putin is doing just about the same as Hitler.’
The prince’s extraordinary intervention is certain to cause international controversy.
It is likely to be seen as a criticism of the West for failing to confront Mr Putin over his seizure of Crimea. The annexation was the first by a major power in Europe since 1945.
Observers have compared the crisis in Ukraine with Hitler’s takeovers of Czechoslovakia and Poland.
They have pointed to the similar use of disguised special forces to stir up tensions in disputed areas.
Charles, who is scheduled to meet Mr Putin at the D-Day commemorations in France on June 6, made his well-intentioned but unguarded comment during a visit to the Canadian Museum of Immigration in Halifax, Nova Scotia.
The prince is on a whistlestop tour of Canada with the Duchess of Cornwall – they have 41 engagements in just three and a half days.
On Monday, the pair paid a heartfelt tribute to Second World War veterans and their families over tea at the museum in Halifax’s docks.
He was introduced to Marienne Ferguson, a museum volunteer who fled to Canada with her Jewish family when she was just 13.
The 78-year-old was born in what is now the Polish city of Gdansk, a key flashpoint in the Second World War.
A free city under the terms of the Versailles Treaty after the end of the Great War, it was seized by the Nazis on the first day of fighting in 1939.