If proof were ever needed that it is never too late to make a major impact, Harry Leslie Smith, who has died aged 95, surely offers it. He was 91 when his bestselling memoir-cum-polemic in defence of the welfare state, Harry’s Last Stand (2014), was published, winning him a mass following in Britain’s ascendant left and beyond.

Following the book’s publication, he was invited to address that year’s Labour party conference before a speech by the then shadow health secretary, Andy Burnham. His passionate denunciation of benefits cuts and austerity – including the line “Mr Cameron, keep your mitts off my NHS!” – made headline news.

Harry became a regular commentator in newspapers, a fixture at speaking events in both Britain and Canada, and a prominent champion of the British Labour party. In the build-up to the 2015 general election, he recorded a party political broadcast for Labour on the NHS, and during the campaign he toured constituencies to drum up support for the party.

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After the Tory victory, Harry endorsed Jeremy Corbyn for the Labour leadership, declaring it was because he wanted “my grandchildren’s generation to have a fighting chance for a decent and meaningful life free of austerity”.

Harry became one of the biggest social media stars in British politics. Within several years, he had sent more than 80,000 tweets and accrued over quarter of a million followers. His widely shared tweets were on a variety of topics: fighting austerity and privatisation, opposing western military interventions, and challenging racism and fascism. He was increasingly preoccupied with rising xenophobia, as demonstrated by the increasing popularity of Nigel Farage and Donald Trump, and saw disturbing parallels with the rise of interwar fascism.

Facebook Twitter Pinterest Harry Leslie Smith served in the RAF during the second world war

He campaigned for Remain in the 2016 EU referendum campaign, arguing that it was Tory austerity rather than EU migrants that was responsible for Britain’s social ills. On the eve of the vote, he warned that “the trials and tribulations Britain faces today are as great as the ones my generation faced and overcame more than 70 years ago”.

Drawing on his wartime experi- ences, he took up the cause of refugees and made visits to the Calais refugee camp. “I have seen camps like the Jungle before – at the end of the war,” he said. “But back then, there was a desire among ordinary citizens and their leaders to alleviate the plight of refugees. Today it is different.”

That Harry became a conscience of the British labour movement later in his life was influenced by his earliest experiences. Born into an impoverished family in Barnsley, South Yorkshire, when he was three Harry experienced the loss of his elder sister, Marion, who died of tuberculosis in a workhouse. His family were too poor to afford a funeral, and she was buried in an unmarked paupers’ grave. His childhood was marked by hunger and abject squalor.

Forced to move to Bradford and then Halifax because of hardship, his father, Albert, a miner, was severely injured in a mining accident. His mother, Lillian (nee Dean), left her husband for another man, who flew into violent alcohol-fuelled rages: he recalled jumping on his back to stop him beating her.

Facebook Twitter Pinterest Harry’s Last Stand was published in 2014. Photograph: Jonny White/Alamy

Harry worked as a barrowboy to help feed his family. By the outbreak of the second world war, he was the 16-year-old manager of a Halifax grocery store. In 1941, he signed up to the Royal Air Force as a wireless operator, and joined British armed forces as they crossed into Belgium, the Netherlands, and finally Germany. Seeing the ruins of Hamburg was a salutary experience: “the stench of death was just unbelievable,” he recalled.

It was in occupied Hamburg that he met his wife, Friede (nee Edelmann), while she was bartering for food. British soldiers were forbidden from going out in public or cohabiting with German women: he would walk 50 paces behind her to avoid being stopped by military police. Although the House of Lords repealed a law banning marriage with citizens of former hostile states in 1946, the stigma and obstacles remained, and it took more than a year before they were married in a German church.

As millions of soldiers waited to be demobilised, Harry recalled jubilation – “dancing and drinking” – when Clement Attlee’s Labour party won a landslide victory in 1945, with his comrades inspired by commitments to create a national health service and build decent, affordable housing. After his service ended in 1948, Harry returned to Yorkshire. However, postwar Britain was not an easy place for him and his German wife to settle in, and so in 1953 they emigrated to Canada, settling in Toronto, and later Harwood, Ontario. He worked for an oriental rug company, selling and helping to design rugs, and Friede became a shipping clerk.

After Friede died of cancer in 1999, Harry backpacked around Europe, visiting places he had known during the war. His youngest son, Peter, who had schizophrenia, died of idiopathic pulmonary fibrosis a decade later. Harry began writing, and self-published his first four books, all memoirs, one of which was republished as Love Among the Ruins in 2015, but it was Harry’s Last Stand that won him his devoted following.

For the last 20 years, Harry divided his time between the UK and Canada. After he was taken ill following a fall in Canada, there was an outpouring of support and solidarity on social media on both sides of the Atlantic.

He is survived by his sons Michael and John, and by his two grandchildren.

• Harry Leslie Smith, writer and activist, born 25 February 1923; died 28 November 2018