Lawyer and father of former Prime Minister Tony Blair;

Born: August 1923; Died: November 17, 2012.

Leo Blair, who has died aged 89, was the father of the former Labour Prime Minister and a man whose complicated upbringing in Glasgow, and changing political loyalties that took in Clydeside Communism and Conservatism, may well explain the career of Labour's most centrist ever Prime Minister. The relationship between father and son was often defined by contrast: Leo wanted to be a Tory MP, his son was a Labour man; Leo was a militant atheist, his son was a man of faith, although for many years that was in secret.

The truth is Leo Blair should never have been brought up in Glasgow at all. He was born in Filey in Yorkshire but this was the 1920s and his parents, Celia Rideway and Charles Parsons, were two of those most shocking of things: unmarried actors. While on tour in Glasgow, they decided they could no longer bring up their young son and gave him up to a Clydeside ship worker James Blair and his wife Mary.

Both his foster parents were firebrand socialists. Not long after Tony Blair's first election victory in 1997, a picture emerged of Mary from the 1920s standing on a campaign lorry emblazoned with the slogan "Peace and Socialism is Our Aim". She also had a fierce commitment to Leo, which did not always express itself in positive ways. When his birth parents married and asked for Leo back, Mary, who had two miscarriages and feared she would never have children of her own, refused and threatened to kill herself. She also burned all Leo's mementos of his real parents while he was away in the army during the Second World War and told his parents he had been killed.

Leo's upbringing was in Golspie Street in Govan, where he went to Govan High School. Many years later his son Tony, then Prime Minister, returned to the street looking for some physical clue to his father's life there but nothing was left. When Leo left school in the 1930s he worked as a clerk for Glasgow Corporation but also embraced politics, joining the Scottish Young Communist League.

But his time in the army, and his ambitions to become a lawyer, began to soften and change his radicalism. By the time he became a lecturer at Durham University, he had ambitions to become a Tory MP and was a particular admirer of Margaret Thatcher. "He was keen on the Thatcher revolution," Tony once said. "That is why I have always understood what attracted people to that."

Certainly, Leo was an example of self-made success. He left school at 14 but returned to education when he was 23, shortly after getting married to Hazel Corscadden, the stepdaughter of a Glasgow butcher. They had two sons, first Bill, then Tony, who was born in 1953. Both boys were later sent to Fettes College in Edinburgh.

As Leo's law career took off, the family decided to emigrate to Australia where Leo worked for a time as a lecturer in Adelaide. Tony's brother Bill remembers their grandmother waving at them from the front door of her house in Glasgow.

After three years, the family returned to the UK where Leo had secured a lecturing job at Durham University. He was also starting to think seriously about becoming an MP when he suffered an unexpected stroke.

Some commentators have suggested it was at this moment that ambition was transferred from father to son. Whether that is true or not, it was a difficult time for the family. Five years later, Hazel was diagnosed with thyroid cancer and died in 1975.

Leo fully recovered from the stroke, remarried and moved to Shropshire. From there, Leo could also watch the progress of his younger son's political career. By 1995, Tony was Labour leader which prompted another political conversion by his father. The man who had once been a member of the Communist Party and had ambitions to be a Tory MP now joined Labour at the age of 71.

The publicity also led to a new chapter in Leo's family history. His biological half sister Pauline, by then 90, contacted the Blairs and a reunion was arranged. It was the first time the family had been in touch since before the war and Tony was there to see it.

Leo Blair is survived by his two sons. He was predeceased by both wives.