Kevin Keiley, from Worthing, West Sussex, selected the @sussexroyal handle when he joined the photo-sharing website because he lives in the county and supports Reading Football Club, whose nickname is the Royals.
But Harry and Meghan, who arrived on the platform this week, have now been given the handle – and Mr Keiley said nobody asked him about it.
“All they needed to do was tell me over email what is happening, that the royals want your name and I would’ve given it, but no, they went behind my back and did it without telling me,” he told ITV.
Mr Keiley said he only became aware of the switch when his son sent him a jokey text message about the fact the royal couple were posting from his old account.
When he logged in, he found his handle – which he had owned “for years” – had been changed to @sussexroyal.
Instagram said the change was in line with its policy which allows it to reassign accounts if users are inactive, though Mr Keiley said he was using it to view other people’s content and like posts.
Mr Keiley does, though, still have possession of the @sussexroyal Twitter handle – and is seeing the publicity pay off.
He told Radio 1 Newsbeat: “I can tell you that Twitter’s gone mental. I used to have four followers and in 24 hours I’ve gone up to about 198.
“What I’m trying to do is keep tweeting, therefore they can’t take it if it’s active. I don’t know what the royals are planning down the line.”
Harry and Meghan became the fastest users to a million followers after they launched their Instagram account on Tuesday.