Grant Harrold, who worked for Charles from 2004 to 2011, added that the Duke of Sussex "get along quite well" with current US President Joe Biden and that Trump has previously shown his support to the Crown, however, Harry "obviously has got reservations".
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"Oh, I don't know," he said. "You'll have to tell me. You just have to tell me. You would have thought they would have known this a long time ago."
Now Mr Harrold told GB News: "The Sussexes possibly could fear Trump coming into power because there could be an investigation which could result in Prince Harry coming back to the UK.
"But you'd like to think that if there was something wrong, if the visa application had been filled out incorrectly, that would have already been addressed in the past and not when Trump comes to power.
"From what it appears, he obviously gets along quite well with Biden and in the past I think Trump has very much kind of spoke of in a way that shows he obviously supports the Crown and the late Queen and The King."
The former royal butler added that Harry has "reservations and that "deep down he probably does fear if Biden comes out of power and Trump goes into power."
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Donald Trump said Prince Harry could face consequences if he lied on his US visa application (Image: Getty)
Mr Harrold explained: "As a member of the Royal Family you do get certain privileges - but they shouldn't be making things that are not fair for other people.
"I remember when I was in the [Royal] Household, they take your passports and they organise your visas, your travel, all that kind of thing.
"So it's something that's normally done by the household, but now that he's doing everything on his own."