Captain Sir Tom Moore has been admitted to hospital with coronavirus, his daughter said.
The 100-year-old, who raised almost £33m for the NHS, was taken to Bedford Hospital after requiring help with his breathing, Hannah Ingram-Moore said on Twitter.
She said he had been treated for pneumonia over the past few weeks and last week tested positive for Covid-19.
Mrs Ingram-Moore said her father was not in intensive care.
A spokeswoman for the family said Capt Sir Tom had not yet received the Covid-19 vaccine due to the medication he was on for pneumonia.
Prime Minister Boris Johnson tweeted: "My thoughts are very much with @CaptainTomMoore and his family. You've inspired the whole nation, and I know we are all wishing you a full recovery."
The Army veteran, originally from Keighley in West Yorkshire, came to prominence by walking 100 laps of his garden in Marston Moretaine, Bedfordshire, before his 100th birthday during the first national lockdown.
He was knighted by the Queen in July.
In Mrs Ingram-Moore's tweet, she said her father had been at home with the family until Sunday when he "needed additional help with breathing".
She said the medical care he had received in the past few weeks had been "remarkable".
"We know that the wonderful staff at Bedford Hospital will do all they can to make him comfortable and hopefully [he will] return home as soon as possible," she said.
NHS Charities Together, which benefited from the millions raised, said he had been an "inspiration to the country" and had also led to many other people raising money and doing "crazy different things to support the charity and give extra support to the NHS".
Chief executive, Ellie Orton, said: "If it wasn't for him and the remarkable fundraising her has done, we wouldn't be able to [provide] the extra support that we give to the NHS.
"It's been phenomenal, the funds that he has raised are making a significant difference in the NHS right now and these funds are additional to what the NHS and the government are able to give."
She said it was being used for extra mental health support and wellbeing rooms and gardens for NHS staff as well as iPads for patients isolated from their families and bereavement counselling.
There has been an outpouring of well wishes for the centenarian on social media.
The Twitter account for England's national football teams said: "We're very sorry to hear this. We are thinking of you all and hoping Captain Sir Tom makes a full and speedy recovery."
In a tweet, Mayor of London Sadiq Khan thanked the NHS for the care the veteran had received and said he hoped he would have a "speedy recovery" and be "back home with his family soon".
Capt Sir Tom, who was given the honorary title of colonel on his 100th birthday, had initially set out to raise £1,000 for NHS charities by repeatedly walking an 82ft (25m) loop of his garden.
But he eventually raised £32,794,701 from more than 1.5m supporters.