ET has learned the 38-year-old royal contracted the virus during the early months of the pandemic.
A source tells ET that William had "fairly significant symptoms."
According to a recent BBC report, Kensington Palace sources confirmed to the news organization that William was diagnosed with COVID-19 shortly after his father, Prince Charles, was diagnosed with the illness.
William reportedly refrained from revealing his diagnosis out of concern that it would alarm the nation during an already challenging time, according to The Sun.
The prince was also reportedly heard telling someone during a royal engagement, "There were important things going on and I didn’t want to worry anyone."
William followed the precautionary coronavirus guidelines by self-isolating at the family home in Norfolk, Anmer Hall, and was treated by official palace doctors during his battle with the virus.
During the month of April, he continued to carry out his royal duties -- which included two dozen video calls and telephone engagements.
"William was hit pretty hard by the virus," a source told The Sun. "At one stage he was struggling to breathe, so obviously everyone around him was pretty panicked."
The BBC's royal correspondent reported that Kensington Palace may have kept the diagnosis a secret to assuage the public's fears of the pandemic, especially after Charles' public battle with the illness. Additionally, the BBC correspondent explained in the report, "The palace also tries to preserve some privacy for the Royal Family."
"I have to admit, at first I was quite concerned," the father of three noted. "He fits the profile of somebody at the age he's at, which is very risky and so I was a little bit worried."
William noted that despite his initial concerns, he ultimately knew that his father would make it through unscathed.
"But my father has had many chest infections, colds, things like that over the years and so I thought to myself, 'If anybody's going to beat this, it's going to be him,'" William said. "And actually, he was very lucky. He had mild symptoms. I think the hardest thing he found was having to stop [working] and not being able to go get a bit of fresh air and go for a walk. He's a mad walker. Loves his walking, so I think he found it quite difficult especially also I think with his mental health being stuck inside."
For more on the impact of the coronavirus on the royal family, see the video below.
Prince William Says He Was 'Quite Concerned' After Dad Prince Charles Was Diagnosed With Coronavirus
Prince Charles and Camila Share Reunion Pic Ahead of 15th Anniversary Following Coronavirus Isolation