The Prince of Wales "has got a point" when he draws comparisons between Russian president Vladimir Putin and Adolf Hitler, Labour leader Ed Miliband has said.
Mr Miliband said that many Britons shared the Prince's concerns about Mr Putin's actions in Ukraine, which were reportedly voiced in a private conversation with a member of the public during his tour of Canada.
Jewish museum volunteer Marienne Ferguson, 78, told Prince Charles how she and her family fled the Nazis during the Second World War, and said that he responded: "Putin is doing just about the same as Hitler".
His comment prompted one senior Labour MP to suggest he "should abdicate" if he wanted to speak out on political affairs. But Deputy Prime Minister Nick Clegg backed his right to express his opinions and said that there was no requirement for the royals to adopt a "Trappist" vow of silence.
Prime Minister David Cameron declined to comment on the Prince's reported remark, but said: "Of course, everyone is entitled to their private opinions."
Asked three times on BBC Radio 4's World at One what he thought of the comments, Mr Cameron said only: "I'm not going to comment on the private conversations of anyone, least of all Prince Charles."
Charles's remark came during a tour of the Museum of Immigration in Halifax, Nova Scotia, where the Prince and the Duchess of Cornwall paid tribute to Second World War veterans and their families.
After meeting Charles, Mrs Ferguson told the Daily Mail that she told the Prince how she fled to Canada shortly before the Nazis annexed the free city of Danzig - now Gdansk in Poland.
"The Prince said 'And now Putin is doing just about the same as Hitler'," said Mrs Ferguson.
"I must say that I agree with him and am sure a lot of people do.
"But I was very surprised that he made the comment as I know they (members of the Royal Family) aren't meant to say these things.
"I told the Prince that while my family and I were lucky to get a permit to travel, many of my relatives had permits but were unable to get out before the war broke out on September 1.
"They were sent to the concentration camps and died."
The comments came just over a fortnight before Charles is due to meet Mr Putin at the commemorations of the 70th anniversary of the D-Day landings in Normandy on June 6.
He and the Duchess are currently on a four-day tour of Canada.
Mr Miliband told Sky News: "I think lots of people across the country will share Prince Charles's concern about President Putin and his actions in the Ukraine.
"I think it's also the case that Prince Charles should be entitled to have private conversations with an individual and those are private conversations. I'm not going to comment on the detail of those conversations."
Mr Miliband added: "I think he has got a point about President Putin's actions and I think he is absolutely entitled to say that there are real concerns about that."
Mr Clegg said Charles was "free to express himself".
"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," the Deputy Prime Minister told BBC1's Breakfast.
"I think he is entitled to his views. But I don't know whether those were his views because I just don't think providing a running commentary on what were private conversations is useful to anybody.
"I don't know exactly what he did or didn't say in that conversation because he thought it was a private conversation."
Asked if he agreed there were parallels to be drawn between Mr Putin and Hitler, Mr Clegg said: "I'm not going to start comparing one period of European history to another.
"People can make different comparisons from different periods of history if they wish. All I would say is that right now I think the behaviour of Putin is not only menacing to Ukraine but it is very destabilising for Europe more generally.
"That is why we continue to say to the Russians, continue to say to Vladimir Putin: step back, de-escalate. It's not in Russia's interests, let alone anybody else's, to continue ratcheting up this tension."
But Nigel Farage rejected the comparison of Putin with Hitler. The Ukip leader, who has previously expressed his admiration for the Russian president, told Sky News: "I know some people feel that way about Putin. I think there's a difference. The difference, I think, is that right from the very start Hitler was expansionist and we haven't seen much evidence of that until now from Putin, and arguably what's happened in the Ukraine is because he's been poked with a stick by the rest of the world."
Labour MP Mike Gapes, a current member and former chairman of the House of Commons Foreign Affairs Committee, condemned the Prince's comments.
The Ilford South MP said on Twitter: "If Prince Charles wants to make controversial statements on national or international issues, he should abdicate and stand for election.
"In a constitutional monarchy, policy and diplomacy should be conducted by Parliament and government. Monarchy should be seen and not heard."
A spokesman for Clarence House said: "We do not comment on private conversations. But we would like to stress that the Prince of Wales would not seek to make a public political statement during a private conversation."
The comments are not the first time the Prince has commented on political affairs.
Letters he wrote to a number of government departments between 2004 and 2005 are the subject of a legal battle over whether their contents should be disclosed.
The Guardian has been trying to make the letters public under the Freedom of Information Act and in September 2012 the High Court ruled in its favour.
The newspaper said the battle was being fought to shed more light ''on the way the heir to the throne seeks to influence government ministers even though he holds no elected position''.
But Attorney General Dominic Grieve has attempted to block the release of the letters, claiming they undermine the principle of the heir to the throne being politically neutral.
In February the Prince spoke out about the winter flooding on the Somerset Levels, calling it a "tragedy" that "nothing happened for so long".
And in 2010 he attracted criticism for expressing strong opinions on a multimillion-pound plan for Chelsea Barracks in London, telling the prime minister of Qatar - who was chair of the developers - that his ''heart sank'' when he saw the design by architect Lord Rogers.
Lib Dem peer Lord Taverne QC argued that Charles had "abused his position" as a result of his apparent remarks.
Speaking on BBC Radio 4's The World At One programme, he said: "He's not an ordinary private citizen. He is entitled to his private views, but he has abused his position.
"Before that, he has written 27 different letters to ministers on various issues promoting his predilection for quack medicine and causes of that kind.
"You can't have a monarch in a constitutional monarchy who seems to take sides on controversial issues and side with Conservatives or Labour. Whatever one's private views and whatever his or her private views.
"The Putin incident is very serious because this is a case of interfering with Anglo-Russian relations.
"All right, it was a private conversation but journalists were nearby. He should show some discretion, he should have a certain amount of sense on these things, he should realise he has got a separate position as heir of state."
Conservative Party chairman Grant Shapps said it was "great" that the Royal Family had personal opinions on big issues.
And he strongly defended Charles's contact with senior ministers, insisting that he had "never, in any way, inappropriately lobbied me".
"I think it is great that we've got royals who have opinions. They are able to have personal opinions; I don't think we should strip them of those things," he told LBC radio.
"I personally am very comfortable with the idea that we have a Royal Family, that individuals will have their own thoughts, that actually, because the actual power, democracy if you like, is contained within Parliament and politicians and what have you, it doesn't change anything."
Mr Shapps would not say whether he had received letters from Charles or shaped policy based on his interventions.
"If you have a system which is a constitutional monarchy then you expect the monarch to have some communication with his or her ministers," he said.
"It is no secret that, on occasion, ministers meet with either the monarch or with Prince Charles, who increasingly is taking over some of those roles. None of this is a mystery or a surprise.
"In the end, it's for politicians to make decisions, for the public to elect them, but we have a very popular and, I think, very good monarchy in this country and I think we are very greatly strengthened for it.
"One thing I can be absolutely clear about: he has never, in any way, inappropriately lobbied me about anything. We have a discussion and a discussion is a discussion, it's not lobbying."
Prime Minister David Cameron told Channel 4 News: "I am not going to comment on someone's private conversation, least of all Prince Charles."