The Duke of Cambridge, 38, did not reveal he was sick because he didn’t want to alarm the public.
William caught the virus in April after the Prince of Wales and Prime Minister Boris Johnson fell ill.
The Duke was treated by palace physicians and quarantined at the family home of Anmer Hall in Norfolk.
‘There were important things going on and I didn’t want to worry anyone,’ the Duke said at an engagement, according to The Sun.
Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge wear face masks during a visit to Beigel Bake Brick Lane Bakery on September 15, 2020 in London
Prince Charles speaks to the nation whilst recovering from coronavirus
Some 23,254 cases were reported in the last 24 hours – 17.5 per cent more than the 19,790 cases seen last Sunday
A further 162 people have died after testing positive for coronavirus in Britain today, in the highest Sunday rise seen since May
That month, William continued working and made 14 telephone and video calls despite sources revealing he was rocked by the disease.
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.
‘He was determined to fulfil his engagements.’
Following the news that his father and the Prime Minister had both fallen ill, William believed that going public with his sickness would only add to the nation’s anxieties.
As the Queen gave her impassioned ‘We Will Meet Again’ address, the Duke decided it would be better not to add to Britain’s strain, sources said.
And despite the impact the virus had on him, the father-of-three continued to contact frontline NHS workers to tell them how ‘proud’ he was of their commitment.
Indeed, his having caught coronavirus brought home just how terrible the contagion is and that it can strike anybody down, a source told The Sun.
William and Kate with George, seven, Charlotte, five, and Louis, two, leading the nation in the ‘Clap for Carers’ in April
On April 16, he opened Birmingham’s Nightingale Hospital via video link and praised staff there for their ‘wonderful example.’
Just days later he appeared on Comic Relief in a pre-recorded sketch alongside Stephen Fry.
He was also filmed outside Anmer Hall leading Kate, 38, and their children George, 7, Charlotte, 5, and Louis, 2, in the ‘Clap for Carers.’
The news comes as William and Kate took the children on a family holiday to a modest stone cottage on the Isles of Scilly for a well-deserved break.
William talks to a patient participating in a COVID-19 vaccine trial at Oxford Vaccine Group’s facility at Churchill Hospital in Oxford, west of London on June 24, 2020
William meets with scientists during a visit to the manufacturing laboratory where a vaccine against the novel coronavirus COVID-19 has been produced at the Oxford Vaccine Group’s facility at the Churchill Hospital in Oxford, west of London on June 24, 2020
Prince William, President of The Royal Marsden NHS Foundation Trust, during a visit to The Royal Marsden in Sutton to meet staff and patient on October 21
Kate and William met six representatives of the NHS to discuss their work during the pandemic and to present them with the special recognition award at Britain’s oldest hospital, St Bartholomew’s in the City of London.
They also offered their support to TV presenter Kate Garraway whose husband Derek is still seriously ill in hospital after contracting Covid-19 in March. Kate introduced the royal couple to the members of the NHS.
The presentation was filmed for the climax of the Daily Mirror Pride of Britain Awards 2020, in partnership with TSB, and was broadcast on Sunday night on ITV.
Thousands of members of the public nominated all the frontline NHS heroes to receive a special recognition Pride of Britain award.
The MailOnline has contacted Kensington Palace for comment.