He was a friend of Arthur Scargill and, from the early 1970s, a committed member of the Communist Party.
But now the Leeds University lecturer has been accused of going several steps further and working as an "agent of influence" for the Stasi, the East German secret police.
Last night he was totally unrepentant about his role. He claimed that, as an avowed Communist, he was doing no more than obeying his conscience in providing information on the political scene in Britain to his comrades in East Germany.
Professor Allen is not accused of passing on state secrets in the classic spy manner but rather of being an "agent of influence".
By this the East Germans and the Russians meant someone they believed to have the ability to influence events in Britain in a manner beneficial to them.
In the 1960s and 1970s Allen was an influential figure in the powerful trade union movement. His area of academic expertise was Labour and union history, and his passion was the miners. It was no surprise that he was to become a friend and confidant of the young Arthur Scargill, then himself a member of the Communist Party.
There is little doubt that he was a highly respected figure on the left, boasting working-class origins and academic pedigree. Later in life he even returned to his bricklaying origins, taking up dry-stone-walling as a hobby.
Professor Allen was born in 1923 in Wales. He left school at 15 to train as a bricklayer, served with the RAF during the war and, in 1946, went to the London School of Economics, where he eventually gained a PhD. In 1959 he was given a lectureship at Leeds University and he became a professor in 1973.
He was involved with the CND from its earliest stages, taking part in the first Aldermaston march, in 1958. He was firmly in the unilateralist camp, which believed that Britain should abandon its own nuclear deterrent, whatever the stance of the Soviet bloc.
In one of the more bizarre episodes of his life, he was arrested in Nigeria in 1964 and accused of plotting to overthrow the government. During his third visit to the country he was arrested in Lagos, the capital, and charged with "managing an unlawful society for the purpose of overthrowing the government by military means" - a charge that carried a seven-year jail sentence. His arrest took place during the height of a nationwide strike over the government's rejection of demands for wage increases. He had visited the country on a sabbatical from Leeds University and was ostensibly gathering material for a book on trade unionism.
Professor Allen subsequently went on hunger strike for eight days, complaining about his treatment in custody, where he was detained in solitary confinement. The charge was later downgraded to sedition and conspiracy involving the circulation of a document urging Nigerian Marxists to "prepare for revolution".
After being granted bail, Professor Allen was caught trying to escape from Nigeria, wearing a turban in an attempt to disguise himself as a local tribesman. He was reportedly exposed when the turban slipped off his head, revealing his white skin. He was eventually sentenced to one year in prison with hard labour but served only four months.
In the aftermath of the miners' strike, it emerged that Professor Allen had travelled to Eastern Europe and had helped raise money in the Soviet Union, East Germany and Hungary for the benefit of the miners. He came close to leading CND when, in 1985, he lost to Joan Ruddock in the battle for the chair of the movement. In 1987 he published The Russians are Coming, which looked at Western attitudes to Russia, then in its glasnost period.
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