If Prince Charles wants to make deeply political speeches to the European Parliament then he should renounce his position as heir to the throne and run for election to it.
What he should not do is abuse that privileged position by offering his thoughts on controversial issues to anyone who will listen.
It's nothing to do with whether he is right or wrong (although he is dangerously wrong on homeopathy and other quackery). It's his inability to stick to the deal under which we have a monarcy - and so keep his trap shut.
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The man is simply not fit to be monarch.
UPDATE: One of my commenters says it's all very well but I wouldn't have said all this if I agreed with Charles. Small problem there: I do agree with him on climate change. But as I made clear, this has nothing to with whether he is right or wrong.