Prince William contracted coronavirus in April but kept it secret to avoid causing alarm, it is being reported.
His illness followed that of his father, Prince Charles, and UK Prime Minister Boris Johnson, influencing the Duke's decision not to go public at a time of much tension in the UK.
"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.
"He was determined to fulfil his engagements."
These engagements extended to 14 telephone and video calls during April.
They included calling NHS staff, opening the Nightingale Hospital in Birmingham, and appearing in a Comic Relief sketch with British actor, author and polymath Stephen Fry.
He was also filmed clapping for carers outside his Anmer Hall home in Norfolk.
The shock news of the Duke's April diagnosis comes just 24 hours after the UK was put into into a second lockdown by Mr Johnson.
And earlier on Saturday, Cabinet Office Minister Michael Gove told Sky News that the restrictions could be extended beyond the slated December 2 finish date.