It is one of the choicest pieces of journalistic dinner party general knowledge that the filthy right-wing Daily Mail was officially a fascist newspaper in the 1930s. The paper was burned on the streets after running the headline "Hurrah for the Blackshirts" and backing Oswald Mosley's plan to make himself Britain's equivalent of Adolf Hitler. No surprise then, so the conversational gambit goes, that the Mail is still beating up on asylum seekers today.

What is less well known is that the Mail's former stablemate the Daily Mirror was just as pro-fascist. On Monday, 22 January, 1934 the Mirror ran the headline "Give the Blackshirts a helping hand". The paper went one further than the Mail, urging readers to join Mosley's British Union of Fascists, and giving the address to which to send membership applications.

"As a purely British organisation, the Blackshirts will respect those principles of tolerance which are traditional in British politics," the Mirror told readers, complaining that "timid alarmists" had "been whimpering that the rapid growth in numbers of the British Blackshirts is preparing the way for a system of rulership by means of steel whips and concentration camps".

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This was nonsense, the Mirror said, the result of ignorance of the reality of "Blackshirt government" in Hitler's Germany: "The notion that a permanent reign of terror exists there has been evolved entirely from their own morbid imaginations, fed by sensational propaganda from opponents of the party now in power."

The paper added that anyone who had visited Germany or Mussolini's Italy "would find that the mood of the vast majority of their inhabitants was not cowed submission but confident enthusiasm."

The Mirror's Sunday sister paper, then known as The Pictorial, followed up with a Hello!-style picture essay showing uniformed blackshirt paramilitaries playing table tennis and enjoying a sing-song around the piano while off duty inside the Black House, Mosley's barracks-cum-dungeon on London's King's Road.

The Mirror and the Pictorial also planned a photographic beauty contest aimed at finding Britain's prettiest woman fascist - though Mosley personally objected to this, saying the paper was trivialising his movement.

The author of the Mirror's "helping hand" article was Harold Harmsworth, the first Lord Rothermere, great grandfather of the current Daily Mail proprietor. Rothermere had inherited both papers from his older brother Lord Northcliffe, but had slowly sold off shares in the Mirror, enabling him to invest in the more profitable Mail. Surprisingly, perhaps, when the Mirror piece was published, he no longer owned the paper. But he still held considerable sway over the paper's board of directors, which he had appointed, including editorial director Harry Guy "Bart" Bartholomew - the man credited with creating the modern tabloid Mirror - and Rothermere's nephew Cecil King, who was to run the paper in its glory years of the 1950s and 1960s.

The change of ownership did not at first change the paper's pro-fascist editorial stance. And when the change came it had more to do with money than ideology. Rothermere's right-wing propaganda had badly hit the paper's sales. Bartholomew and King's solution was to re-launch the paper as a New York-style tabloid aimed at a working-class audience.

"Our best hope," King later wrote in his memoirs, "was to appeal to young, working-class men and women... If this was the aim, the politics had to be made to match. In the depression of the thirties, there was no future in preaching right-wing politics to young people who were in the lowest income bracket."

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When the political shift in the Mirror came it was cautious. The paper backed the Conservative leader Stanley Baldwin in the 1935 election, and then gradually adopted an anti-appeasement policy. But politics was far less important in the re-launched, tabloid Mirror. The paper cut its politics coverage by half and vastly increased its sport reporting, shock-horror pictures, lurid crime tales, cartoons, human-interest material and pin-up pictures.

King and Bartholomew's American-style tabloid formula - put into action with enormous panache by legendary Welsh tabloid feature-writer Hugh Cudlipp - doubled the circulation to 1.5 million by 1939.

During the war - in true tabloid style - the Mirror became super-patriotic, and won for itself the reputation of being "the soldiers' paper". Much of the paper's radical reputation rested on its demagogic attacks on the "Colonel Blimp" Conservative politicians and upper-class army officers who made such a mess of the war effort in its early stages.

But the idea of the 1930s Mirror as a great champion of the anti-Nazi cause is largely mythical. And there is no indication that Cecil King ever changed his politics. King remained an admirer of Oswald Mosley, announcing in his memoirs that Mosley had been "the outstanding politician of his generation" and that his only mistake was to have "chosen the wrong side during the war".

After the war, Cecil King came to run the Mirror with as much autocratic power as any proprietor. But wisely, he left the contents of the paper to Cudlipp, the man with the common touch. Despite the paper's reputation for supporting all things socially radical in the 1950s and 1960s, its editorial support for Labour was lukewarm.

King still felt the Harmsworth-Rothermere blood coursing through his veins and loathed Labour's post-war leaders Attlee ("a complete drip") and Gaitskell ("a vain man without substance or principle"). He warmed at first to Harold Wilson, mainly because Wilson had promised to take the UK into the European Common Market.

By the 1960s the theme of a "united Europe" standing between what the Mosleyites saw as a Mongolian-Asiatic Russia and a Jewish-Negro America had become an obsession with the exiled Mosley and also with King. Dumbfounded hacks at the Mirror were required to write article after article setting out the plan for "Nation Europa", which were then foisted on a mostly baffled Mirror readership.

In 1968, after Wilson dragged his feet on Europe, and at the height of a run on the pound, King commandeered the front page of the Mirror to demand Wilson's removal from office. At the same time, amid talk of a military coup, King held a meeting with Mosley at his mansion outside Paris, sounding him out as a possible member of a "government of national unity".

Peter Stephens, the Mirror's Paris correspondent sent a report back to Cudlipp in London (now contained in Cudlipp's private archive at Cardiff University) reporting that King had said that Mosley was still "an extremely brilliant man" who could "still make a useful contribution" to the running of the country. Stephens, astonished, had asked: "You are surely not thinking of including him in your replacement government?" King had replied: "Why not? People have forgotten about his past."

In the event - after some further meetings with military officers and an audience with the potential figurehead Lord Mountbatten - King's plan for the establishment of a Mirror-led military dictatorship fizzled out and was written off as an act of insanity.

King's role in the 1968 "coup that never was" is still controversial. But the fact remains that for much of the Mirror's admittedly brilliant 100-year reign as the self-proclaimed "Newspaper of the Century", it had a dark side which many now prefer discreetly to forget.

Chris Horrie is author of 'Tabloid Nation: From the birth of the Mirror to the death of the tabloid newspaper'; Andre Deutsch, £17.99