Prince William battled coronavirus in April and kept it secret so he didn't alarm the nation, according to reports.
The Duke of Cambridge followed the government guidelines by self isolating at home but was said to have been "hit pretty hard by the virus".
He was treated by palace doctors but kept his diagnosis secret because he did not want to cause alarm, reports said.
Prince William reportedly did not want to worry people, following news of his dad's diagnosis - as well as Boris Johnson's.
A source told the Sun : "William was hit pretty hard by the virus - it really knocked him for six.
"At one stage he was struggling to breathe, so obviously everyone around him was pretty panicked.
"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 though."
Prince William is believed to have suffered from Covid-19 days after his dad Prince Charles also battled the virus.
He is said to have still managed to do 14 telephone and video call engagements during April.
A source said he was determined to carry out his engagements.
The news comes after earlier in the week Prince William told TV presenter Kate Garraway he and his wife wanted to give her a hug after hearing of her family’s Covid nightmare.
The Duke and Duchess of Cambridge were visibly moved as the Good Morning Britain host told how husband Derek is still seriously ill in hospital seven months after being struck down with coronavirus.
They listened intently as she discussed the impact that the disease has had on her family and how their children Darcey, 14, and Billy, 11, are still not allowed to see him because covid rules only allow one visitor is allowed for each patient.
William said: “You are doing amazingly though. If it wasn’t for social distancing we would both give you a hug.”
Kate also discussed how William’s father, Prince Charles had been diagnosed with Covid at the same time as her husband in March.
William said: “Yes, my father had it really early on.”