Captain Sir Tom Moore has died with coronavirus.
The 100-year-old, who raised almost £33m for NHS charities by walking laps of his garden, was admitted to Bedford Hospital on Sunday.
The Queen led tributes to Capt Sir Tom, "recognising the inspiration he provided for the whole nation and others across the world".
His daughters said they "shared laughter and tears" with their father in their final few hours together.
Announcing his death, Hannah Ingram-Moore and Lucy Teixeira said the last year of their father's life had been "nothing short of remarkable".
He tested positive for Covid-19 last week. His family said due to other medication he was receiving for pneumonia, he was unable to be vaccinated.
The Army veteran won the nation's hearts by walking 100 laps of his garden in Marston Moretaine in Bedfordshire last year during the first lockdown, raising money for NHS Charities Together.
He was credited with lifting the nation's spirits and his saying "Tomorrow will be a good day" trended on social media.
He was knighted by the Queen in July in a special ceremony at Windsor Castle.
A Buckingham Palace spokeswoman said: "Her Majesty very much enjoyed meeting Capt Sir Tom and his family at Windsor last year. Her thoughts, and those of the royal family, are with them, recognising the inspiration he provided for the whole nation and others across the world."
In a statement, Prime Minister Boris Johnson said: "Captain Sir Tom Moore was a hero in the truest sense of the word. In the dark days of the Second World War he fought for freedom and in the face of this country's deepest post-war crisis he united us all, he cheered us all up, and he embodied the triumph of the human spirit.
"He became not just a national inspiration but a beacon of hope for the world. Our thoughts are with his daughter Hannah and all his family."
The flag above 10 Downing Street has been flying at half-mast in tribute and Mr Johnson has spoken to Mrs Ingram-Moore to offer his condolences.
A tweet from the White House said: "We join the United Kingdom and the world in [honouring] the memory of Captain Sir Tom Moore, who inspired millions through his life and his actions."
Labour leader Sir Keir Starmer tweeted: "This is incredibly sad news. Captain Tom Moore put others first at a time of national crisis and was a beacon of hope for millions. Britain has lost a hero."
The daughters' statement said: "It is with great sadness that we announce the death of our dear father.
"We are so grateful that we were with him during the last hours of his life; Hannah, Benjie and Georgia by his bedside and Lucy on FaceTime.
"We spent hours chatting to him, reminiscing about our childhood and our wonderful mother. We shared laughter and tears together.
"The last year of our father's life was nothing short of remarkable. He was rejuvenated and experienced things he'd only ever dreamed of.
"Whilst he'd been in so many hearts for just a short time, he was an incredible father and grandfather, and he will stay alive in our hearts forever."
Capt Sir Tom's daughters said the care he received from the NHS was "extraordinary".
They said staff had been "unfalteringly professional, kind and compassionate and have given us many more years with him than we ever would have imagined".
Capt Sir Tom joined the Army at the beginning of World War Two, serving in India and Myanmar, then known as Burma.
He was originally from Keighley in West Yorkshire and among the local tributes being paid was Robbie Moore MP who said the town had "lost one of its finest today".
He was made an honorary colonel of the Army Foundation College in Harrogate on his 100th birthday.
Capt Sir Tom had initially set out to raise £1,000 for NHS charities by walking 82ft (25m)-loops of his garden.
But he eventually raised £32,794,701 from more than 1.5 million supporters.
NHS Charities Together said that would rise to £39m when Gift Aid was taken into account.
Ellie Orton, chief executive, said the funds raised by Capt Sir Tom had "reached the length and breadth of the UK through every one of our 241 member charities".
She said he was "a one-off and he leaves the world a better place".
Ruth May, chief nursing officer for NHS England, said Capt Sir Tom Moore "has been the model of all that has been good about our country's response to Covid-19".
She said in a statement "for me his biggest achievement and most important contribution to helping my fellow nurses, doctors and all those in the NHS responding to coronavirus, has been how he brought the country together and gave us all a boost when we most needed it".
Fellow charity fundraiser Dabirul Choudhury, who was 100 years old when he raised more than £150,000 for coronavirus relief by walking while fasting for Ramadan, paid tribute to Capt Sir Tom.
"If you want to help mankind you should keep yourself very fit, fit, fit," he said.
Mr Choudrey's son Atique said Capt Sir Tom had "left a massive legacy that will follow on through for generations", adding "even now, my father hasn't actually eaten since he's heard about the news [of Capt Sir Tom's death]".
Capt Sir Tom became the oldest person to have a UK number one single when he recorded You'll Never Walk Alone with Michael Ball last year.
The singer said on Twitter: "A wonderful life so well lived and a hero and fighter to the very end. So very sad".