Prince William had COVID-19 in April — and “struggled to breathe’’ at one point, sending royal aides into a terrified panic, a report said Sunday.

The British royal, 38, fought to keep the news of his illness out of the press because dad Prince Charles and the country’s prime minister, Boris Johnson, had both just come down with the deadly contagion and William was concerned how his illness would worry the nation even more, sources told The Sun.

“William was hit pretty hard by the virus — it really knocked him for six,’’ a source told the outlet.

“At one stage he was struggling to breathe, so obviously everyone around him was pretty panicked.”

But “after seeing medics and testing positive, which was obviously quite a shock given how fit and healthy he is, William was determined it should be business as usual,’’ the source said. “He was determined to fulfill his engagements.”

The prince, who is next in line to succeed his father to the throne, was treated by palace doctors and properly quarantined at his family’s home, Anmer Hall, in Norfolk — all while carrying out work phone calls and video chats, the outlet said.

The married dad of three later told a source at an event that he kept his illness quiet because “there were important things going on, and I didn’t want to worry anyone,’’ The Sun reported.

Now, “as a result of his own experiences, he realizes absolutely anyone can catch this awful disease and knows how imperative it is that we all take this second lockdown seriously,’’ a source said.

The Palace did not respond to The Sun’s request for comment.