British media reports said several witnesses heard the prince make the comment on May 19 to Marienne Ferguson, a 78-year-old who fled the Nazis aged 13 and lost family members in the Holocaust.
Ferguson told the media that Charles, heir to the British throne, said, "...Putin is doing just about the same as Hitler."
Charles met Ferguson during a tour of the Canadian Museum of Immigration in Halifax, Nova Scotia, as part of a four-day trip to Canada with his wife, Camilla.
He is due to meet with Putin during D-Day commemorations in France on June 6.
Clarence House, the official residence of Prince Charles, said it would not comment on a private conversation.
Russia's MK website reacts to Prince Charles' alleged Putin remarks:he "risks complicating UK-Rus relations which have already clouded over"
— Steve Rosenberg (@BBCSteveR) May 21, 2014
Today's Komsomolka has this diagram of Medvedev's desk. Number 11 screen right is the phone for speaking to Putin. pic.twitter.com/wiYS8y5wb1
— Steve Rosenberg (@BBCSteveR) May 21, 2014
Komsomolka interview with Medvedev is hardhitting: "Why are there no flowers in your office?" "What's that large rhinocerous on your desk?"
— Steve Rosenberg (@BBCSteveR) May 21, 2014
Medvedev asked if he'll give up his "US gadgets" over tension with USA. "What's that got to do with the Iphone?" he says. That's a no, then.
— Steve Rosenberg (@BBCSteveR) May 21, 2014
Ukraine bans entry to @RT_com crew sent to cover presidential election http://t.co/NdH2eXMVWl
— Maxim Eristavi (@MaximEristavi) May 21, 2014
Militia claims that they have arrested 7 shooters responsible for the 20 February killings on #euromaidan with more to come
— bruce springnote (@BSpringnote) May 21, 2014
Hmm. Akhmetov may have left it too late. @courtneymoscow finds little enthusiasm for protest amongst his workers. http://t.co/2e1YaijEOa
— Roland Oliphant (@RolandOliphant) May 21, 2014
Everyone seems to be detaining the media now, including Ukraine, the separatists AND the Russian Federation https://t.co/o7FHYTfIcd
— Simon Ostrovsky (@SimonOstrovsky) May 21, 2014
The detention on May 18 of Oleg Sidyakin and Marat Saichenko prompted a furious reaction from officials in Moscow, who demanded their immediate release.
But U.S. State Department spokeswoman Jen Psaki said on May 20 that Ukrainian security services reported the two had "manned portable antiaircraft missiles in the trunks of their cars" when they were detained.
Psaki said Washington condemns “the unlawful detention of journalists in any capacity.”
But she said Kyiv is questioning whether the two "were actually journalists."
LifeNews is owned by News Media Holding – a private Russian firm controlled by Kremlin-loyalist Aram Babrelyanov and whose major shareholders include members of Russian President Vladimir Putin’s inner circle.
Putin's press secretary Dmitry Peskov tells Sky News "he knows nothing" about the comment made by Prince Charles.
— Yulia Bragina (@YuliaSkyNews) May 21, 2014
#EuroMaidan six month anniversary today. Here is a good timeline of the events leading to Yanukovych leaving Ukraine. http://t.co/3V8geeJcEq
— Anders Östlund (@andersostlund) May 21, 2014