Chris O'Neill, who declined a royal title in order to be able to retain his dual British-American nationality and continue his work after marrying King Carl XVI Gustaf's daughter Madeleine in 2013, made the comments on his Instagram account following Trump's signing of a controversial executive order last week.
Born in London to a father from the US, O'Neill has lived in New York in the past. He posted his feelings about Trump on his private Instagram account on Sunday, but when his friend, former Valentino honorary president Giancarlo Giammetti shared it on his own account, it became possible for the public to read it, which Swedish tabloid Aftonbladet was quick to pick up on.
"On behalf of all sane Americans, I am very sorry for the actions of our shameful president," O'Neill wrote.
"It is a sad day to be a US citizen. Mr. President, you are a painfully ignorant human being. You will be the sole cause of the violence and hatred you suggest you are protecting us from. You have neither sense nor sensibility, Sir. #reaganyouain't," he added.
The protocol for Swedish royals is to stay neutral on political issues, but O'Neill's lack of a title has allowed him greater freedom than is habitual. He has commented on other political issues like Brexit for example in the past, saying "of course Britain should not leave" in July 2016.