Captain Sir Tom Moore has been admitted to hospital with coronavirus, his daughter has 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: “You’ve inspired the whole nation, and I know we are all wishing you a full recovery.”
The Army veteran 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 December, he went on a family holiday in Barbados after British Airways paid for his flight.
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.
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.”
Health Secretary Matt Hancock also sent his “best wishes”, while Labour leader Sir Keir Starmer said the whole nation was wishing him well, adding: “You’ve been an inspiration to us all throughout this crisis.”
BBC Breakfast presenter Dan Walker posted: “Come on Captain Tom”, while actor and singer Michael Ball – who recorded a charity single with Capt Sir Tom – sent “love and prayers”.
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.