Captain Tom Moore, 100, who won British hearts with a Covid-19 fund raising drive, was hospitalised on Sunday with the virus, his family said.
Moore raised millions of pounds for charity by walking around his garden and then became the oldest man to top the UK music charts with a cover of "You'll Never Walk Alone".
"Over the last few weeks he was being treated for pneumonia and last week tested positive for Covid-19," Moore's daughter Hannah Ingram-Moore said on Twitter.
"He was at home with us until today when he needed additional help with his breathing," she explained, adding that "he is being treated in a ward although he is not in ICU", or an intensive care unit.
Moore was admitted to hospital in Bedford, central England.
The BBC quoted a spokesman for the family as saying that he had not yet been vaccinated for the virus owing to his pneumonia.
Moore, who served in India during World War II, raised £33 million (37 million euros, $45 million) for staff of the state-run National Health Service (NHS) battling the coronavirus outbreak.