Mr Smith, who rose to fame in 2014 after giving a powerful speech at the UK Labour Party’s convention, was hospitalised in Ontario, Canada since November 14.
While his father was unwell, his son John Smith took over his Twitter account, which has more than 254,000 followers around the world.
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The left-wing activist has been flooded with support since his son John began tweeting yesterday about his father being gravely ill. John told Harry’s 239,000 followers that their messages have helped him feel less lonely while keeping vigil
At around 8:30 GMT today, John Smith tweeted: ‘At 3:39 this morning, my dad Harry Leslie Smith died. I am an orphan. #istandwithharry,’.
Hours earlier, he had tweeted saying Harry had ‘a good night’ but appeared to be ‘very tired’.
Mr Smith had been touring the world’s refugee hotspots when he fell ill before his flight back to the UK, his family two weeks ago.
Born in Barnsley, South Yorkshire, Mr Smith served with the RAF in Germany and in recent years campaigned on behalf of the NHS.
Smith survived Britain’s post-World War One recession in abject poverty and spent much of his later life campaigning to ensure others did not experience the same, his son said.
The nonagenarian documented his fight for social justice in a podcast called ‘Harry’s Last Stand,’
RAF veteran Harry Leslie Smith is critically ill in a Canadian hospital. His son John has been tweeting from the 95-year-old’s bedside about his condition following a fall yesterday
Besides healthcare and refugee advocacy, Smith has written several books since turning 87
Besides healthcare and refugee advocacy, Smith has written several books since turning 87.
While offering medical updates, John opened up about the closeness he shared with his father, writing: ‘Since my brother Pete’s death, 9 years ago, I have been at Harry’s side and never absent from him for more than a day.
‘We laughed, we argued, we learned from each other. I became his friend while still letting him remain my dad.’
On hearing that Smith was seriously ill, Jeremy Corbyn said: ‘We will all miss Harry Leslie Smith – he was one of the giants whose shoulders we stand on.
‘A World War Two veteran who dedicated his life to fighting for our National Health Service, a peaceful world and for countries to meet their moral responsibility by welcoming refugees.’
The political commentator grew up in poverty in Barnsley, Yorkshire before serving in the RAF in World War II
Former Labour leader Ed Miliband tweeted: ‘Very sad to hear of the death of Harry Leslie Smith. He was one of a kind who never wavered in his fight for equality and justice. We should all carry his passion, optimism and spirit forward.’
Labour MP Jonathan Ashworth wrote on Twitter: ‘RIP Harry Leslie Smith will always remember this wonderful speech on the NHS you gave at Labour Conference.’
Labour MP Luke Pollard also paid tribute, tweeting: ‘RIP @Harryslaststand. Harry Leslie Smith came to #plymouth ahead of the 2015 General Election. He was old and frail but full of life. His encouragement and positivity was a true inspiration. RIP Harry’