The Duke of Cambridge tested positive for COVID-19 in April, the BBC reported on Monday.
Prince William, 38, caught the virus days after his father Prince Charles tested positive in March.
According to The Sun, the Prince was "struggling to breathe" after being struck with the deadly virus.
Sources told The Sun he kept his diagnosis private to avoid sending the country into panic mode.
"William was hit pretty hard by the virus - it really knocked him for six," a source told The Sun.
"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."
The Duke of Cambridge reportedly carried out 14 telephone and video call engagements in April.