Prince William secretly battled COVID-19 illness in April, just days after his father Prince Charles and British Prime Minister Boris Johnson contracted it, a new report has claimed.

However, Prince William kept his diagnosis a secret as he did not want to alarm the public. He continued his virtual engagements with charities even though he was "struggling to breathe," reports The Sun.

According to the report, William told one observer at an engagement: "There were important things going on and I didn't want to worry anyone."

The second-in-line to the British throne was treated by palace doctors while he isolated at his country home Anmer Hall in Norfolk, where his wife Kate Middleton and children Prince George, Princess Charlotte, and Prince Louis were also staying.

A source told the outlet that William was hit pretty hard by the virus, but carried out his royal duties routinely. The 38-year-old held as many as 14 telephone and video call engagements during April.

"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," the insider revealed.

"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," the source added.

The insider explained that the public was already alarmed by the COVID-19 diagnosis of the heir apparent Prince Charles and PM Boris Johnson, so William kept it secret as he didn't want to send them into panic mode.

"The Queen delivered her 'We Will Meet Again' address, and he just didn't want to worry people. He felt there were more important things going on in the country. But as a result of his own experiences, he realises absolutely anyone can catch this awful disease - and knows how imperative it is that we all take this second lockdown seriously," the source said.

William's diagnosis with the respiratory disease came just weeks after he cracked an awkward joke about the coronavirus outbreak and how he is perceived as "super-spreader" of the virus.

During a visit to Ireland with wife Kate in March, the British royal asked a paramedic whether he thought coverage of coronavirus was "being a little hyped up" in the media.

"I bet everyone's like 'I've got coronavirus, I'm dying', and you're like 'no, you've just got a cough. Does it seem quite dramatic about coronavirus at the moment? Is it being a little bit hyped up do you think in the media,'" the Duke of Cambridge asked Joe Mooney, an advance paramedic with the National Ambulance Service.

The father-of-three ended up making the conversation even more awkward, as he quipped: "By the way, the Duke and Duchess of Cambridge are spreading coronavirus! Sorry... We're keeping an eye on that, so do tell us if we need to stop."

Prince William
Prince William Getty Images/Richard Stonehouse