Anthony Wedgwood Benn, who has died aged 88, was a gifted and captivating politician, campaigner, orator and diarist who believed in an old-school style of socialism and believed in it all the more the older he became.
This often put him at odds with his own party (he fiercely opposed the modernisation of Labour in the 1980s)but his commitment and the constancy of his beliefs won him admirers in all parties. "Say what you believe and believe what you say," he said. "That's the test."
He famously came from a privileged background but renounced his title so that he could remain in the House of Commons, where he served as an MP for more than 50 years.
By the end of that time he had become frustrated with what he saw as the undermining of the Commons by government and, on his retirement, said, in a joke that for him contained a large element of truth, that he was leaving to spend more time on politics.
He achieved high office (in Cabinet, he ran four departments over 11 years)but was much derided for an ability to be in government and against it at the same time.
He was responsible for some significant decisions (he oversaw the opening of the Post Office Tower and was the minister in charge of Concorde) but he was frustrated in his attempts to lead the Labour Party.
He stood for the leadership twice, once in the 1970s and again in the 1980s, and was defeated; he also stood against Denis Healey for the deputy leadership, leading to much bitterness and division in the party.
He was born Anthony Neil Wedgwood Benn, the second son of William Wedgwood Benn, a minister in early Labour governments, and was educated, in classic English upper-middle class fashion, at Westminster and New College Oxford; he was elected president of the Union in 1947.
His military service was in the RAF as a pilot officer and when, in 1947, he married Caroline De Camp, an heiress, his children were vested by their American grandparents with a substantial trust fund.
His entry into politics came early when he succeeded Sir Stafford Cripps, Clement Attlee's Chancellor, in the constituency of Bristol South-East in 1950. Influential in his selection was Anthony Crosland, his tutor at Oxford, an emblematic figure of the Labour right, who remained a friend. In typical cavalier fashion, Crosland dismissed the rage at Mr Benn during the 1970s by invoking his family's pet name for Mr Benn: Jimmie. "Nothing wrong with Jimmie except he's a bit cracked," he said amiably.
In 1960, Mr Benn's career went off on an interesting tangent. When his father died in 1960, he became, inexorably so it seemed, Viscount Stansgate. But Mr Benn simply refused to go to the Lords, and appealed successfully to his party for support. After a change in the law, he was permitted to remain an MP.
The decision helped his image as a popular politician with a knack for the informal. In the early 1960s before Labour won office, he would sling his coat on the back of a chair, sit on the table and talk with an informality quite unknown in those days of grasped lapels and official manner.
There was a streak of vanity in him too. Called on to open the Post Office Tower in 1965, he made several comparisons with Big Ben. "He wants them to call it Bigger Benn," said an unimpressed journalist.
However, the devotion to technology was real: no gadget or gismo was safe from endorsement and, where possible, demonstration by an enthusiastic minister. His advance to the new Ministry of Technology, these being the white-hot days, was appropriate.
The humiliations of Jim Callaghan's Treasury changed Mr Benn's fortunes and economic failure re-opened the right and left quarrel. Slowly after 1968, Mr Benn, never giving up office, turned from low temperature technocrat into evangelical leftwinger.
He fell among zealots, made speeches to excitable young people in an excitable period and found that they loved him. He began to function as the unofficial leader of the Left, heir to Aneurin Bevan and scourge of the leadership.
Labour's surprise defeat in June 1970 left the party perfectly equipped for renewed hostilities. These began to show with moves to de-select right-wing MPs. Mr Benn, who became party chairman in 1971, applauded this and for a long time, with Labour in opposition and the leadership terrified of what stood behind him, Mr Benn became powerful.
Back in power, economic crisis compelled the Chancellor, Denis Healey, to engage in spending cuts and interest rate increases quite contrary to anything Healey, the Labour Party or Mr Benn wanted but Mr Benn invoked Ramsay MacDonald and the legend of betrayal. The account in his vast, many volumed and entertaining diaries, is, for all its subjectivity, the most vivid there is of this unnerving, if fascinating period. Ironically, Healey, after early mistakes fed by calamitous forecasts, would secure the currency, achieving a three- year voluntary wage policy which worked, helped by the major union leaders who had moved on from Mr Benn's vision, having, as Hugh Scanlon put it, "looked into the abyss".
Mr Benn's actual power in the Cabinet had been reduced as soon as Harold Wilson had felt able to move him from Industry to Energy in June 1975. At Energy, he could give money to coalmining but had to deal privately with the oil companies. His actual effect upon government had been aspirational and transient.
However, his effect upon the Labour Party swelled and was never greater than when, in the wake of Callaghan's blunders, it stumbled into the 1979 election. Mr Benn's strength came from the growing pressure for a revised party constitution. This would reduce the power of MPs, a majority of whom loathed him, and increase that of unions and constituencies.
In 1980, Labour chose as its leader the unelectable Michael Foot. Mr Benn, in whose circle a feverish hope of taking over the party now bubbled up, aimed his pitch at Foot's reluctant deputy, Denis Healey. The contest for this post was recognised as one for the party's future direction but a Benn victory would have precipitated a haemorrhage of talent and seniority bigger than that achieved by the creation of the SDP. It was seen off in the terms of a new local party-friendly constitution, by the narrowest of margins: 0.6 % of the votes cast. Defeat at Bristol in 1983, an election which overall tested left-led Labour to destruction, barred Mr Benn from the leadership election.
But within a year (March 1984), he returned at the Chesterfield by-election. He was back in time for the excitement, demonstration and speech-making of the year-long miners' strike, although he would dwindle comprehensively as a force for power.
He remained an MP until 2001 and, as a voice of the anti-war movement, his popularity expanded beyond his old emotional constituency and, away from the bad temper and personalisation of a power struggle, Mr Benn was a formidable and persuasive advocate. He was courteous, soft spoken, funny and simply better in debate than almost all the competition. Old enmities subsided.
He published several volumes of memoir and diaries, the last of which was A Blaze of Autumn Sunshine: The Last Diaries. Speaking on its publication last year, he said politics had been his life. "I can't think of a better way of spending a life," he said.
He was pre-deceased by his wife Caroline and is survived by his children Hilary, Melissa, Joshua and Stephen.