The 37-year-old royal admitted he felt a ''little bit worried'' about his 71-year-old dad when he fell ill with ''mild symptoms'' of the disease last month but was confident the heir-to-the-throne would pull through.
He told 'BBC Breakfast' via video link: ''At first I was quite concerned - he fits the profile of somebody, at the age he is, who is fairly risky.
''I was a little bit worried, but I thought if anybody is going to beat this, it's going to be him.''
Meanwhile, William has been finding it ''fun'' homeschooling his children - Prince George, six, Princess Charlotte, four, and Prince Louis, 23 months - but his wife Duchess Catherine admitted it has been ''challenging''.
And the duchess confessed to feeling ''guilty'' because she's kept her brood's lessons going throughout the Easter holidays.
She said: ''Don't tell the children, we've actually kept it going through the holidays. I feel very mean.''
The couple appeared on the show to highlight their Every Mind Matters mental health initiative, which aims to help people take simple steps to look after their wellbeing and support others, as they feel ''very strongly'' it is more important now than ever.
William said: ''We felt very strongly that now more than ever this was a vital tool and a vital service that people could easily access at home to guide them and give them just some very basic tips to mind their mental health, to mind their mental wellbeing through this process.
''I think again, staying connected, staying positive and being able to talk to friends and family is so crucial and having just some tips and some ideas as to how to tackle some of these strange feelings and difficult circumstances we're finding ourselves in is really important, just to nudge us through these next few weeks.''
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