Orwell himself wrote of it in his preface to Animal Farm:
https://www.marxists.org/archive/orwell/1945/preface.htm
> Any fair-minded person with journalistic experience will admit that during this war official censorship has not been particularly irksome. We have not been subjected to the kind of totalitarian ʻco-ordinationʼ that it might have been reasonable to expect. The press has some justified grievances, but on the whole the Government has behaved well and has been surprisingly tolerant of minority opinions. The sinister fact about literary censorship in England is that it is largely voluntary.
> Unpopular ideas can be silenced, and inconvenient facts kept dark, without the need for any official ban. Anyone who has lived long in a foreign country will know of instances of sensational items of news – things which on their own merits would get the big headlines – being kept right out of the British press, not because the Government intervened but because of a general tacit agreement that ʻit wouldn't doʼ to mention that particular fact. So far as the daily newspapers go, this is easy to understand. The British press is extremely centralised, and most of it is owned by wealthy men who have every motive to be dishonest on certain important topics. But the same kind of veiled censorship also operates in books and periodicals, as well as in plays, films and radio. At any given moment there is an orthodoxy, a body of ideas which it is assumed that all right-thinking people will accept without question. It is not exactly forbidden to say this, that or the other, but it is ʻnot doneʼ to say it, just as in mid-Victorian times it was ʻnot doneʼ to mention trousers in the presence of a lady. Anyone who challenges the prevailing orthodoxy finds himself silenced with surprising effectiveness. A genuinely unfashionable opinion is almost never given a fair hearing, either in the popular press or in the highbrow periodicals.
> At this moment what is demanded by the prevailing orthodoxy is an uncritical admiration of Soviet Russia. Everyone knows this, nearly everyone acts on it. Any serious criticism of the Soviet régime, any disclosure of facts which the Soviet government would prefer to keep hidden, is next door to unprintable. And this nation-wide conspiracy to flatter our ally takes place, curiously enough, against a background of genuine intellectual tolerance. For though you are not allowed to criticise the Soviet government, at least you are reasonably free to criticise our own. Hardly anyone will print an attack on Stalin, but it is quite safe to attack Churchill, at any rate in books and periodicals. And throughout five years of war, during two or three of which we were fighting for national survival, countless books, pamphlets and articles advocating a compromise peace have been published without interference. More, they have been published without exciting much disapproval. So long as the prestige of the USSR is not involved, the principle of free speech has been reasonably well upheld. There are other forbidden topics, and I shall mention some of them presently, but the prevailing attitude towards the USSR is much the most serious symptom. It is, as it were, spontaneous, and is not due to the action of any pressure group.
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> It is important to distinguish between the kind of censorship that the English literary intelligentsia voluntarily impose upon themselves, and the censorship that can sometimes be enforced by pressure groups. Notoriously, certain topics cannot be discussed because of ʻvested interestsʼ. The best-known case is the patent medicine racket. Again, the Catholic Church has considerable influence in the press and can silence criticism of itself to some extent. A scandal involving a Catholic priest is almost never given publicity, whereas an Anglican priest who gets into trouble (e.g. the Rector of Stiffkey) is headline news. It is very rare for anything of an anti-Catholic tendency to appear on the stage or in a film. Any actor can tell you that a play or film which attacks or makes fun of the Catholic Church is liable to be boycotted in the press and will probably be a failure.
> But this kind of thing is harmless, or at least it is understandable. Any large organisation will look after its own interests as best it can, and overt propaganda is not a thing to object to. One would no more expect the Daily Worker to publicise unfavourable facts about the USSR than one would expect the Catholic Herald to denounce the Pope. But then every thinking person knows the Daily Worker and the Catholic Herald for what they are. What is disquieting is that where the USSR and its policies are concerned one cannot expect intelligent criticism or even, in many cases, plain honesty from Liberal writers and journalists who are under no direct pressure to falsify their opinions. Stalin is sacrosanct and certain aspects of his policy must not be seriously discussed.
> This rule has been almost universally observed since 1941, but it had operated, to a greater extent than is sometimes realised, for ten years earlier than that. Throughout that time, criticism of the Soviet régime from the left could only obtain a hearing with difficulty. There was a huge output of anti-Russian literature, but nearly all of it was from the Conservative angle and manifestly dishonest, out of date and actuated by sordid motives. On the other side there was an equally huge and almost equally dishonest stream of pro-Russian propaganda, and what amounted to a boycott on anyone who tried to discuss all-important questions in a grown-up manner. You could, indeed, publish anti-Russian books, but to do so was to make sure of being ignored or misrepresented by nearly me whole of the highbrow press.
> Both publicly and privately you were warned that it was ʻnot doneʼ. What you said might possibly be true, but it was ʻinopportuneʼ and played into the hands of this or that reactionary interest. This attitude was usually defended on the ground that the international situation, and me urgent need for an Anglo-Russian alliance, demanded it; but it was clear that this was a rationalisation. The English intelligentsia, or a great part of it, had developed a nationalistic loyalty towards the USSR, and in their hearts they felt that to cast any doubt on me wisdom of Stalin was a kind of blasphemy. Events in Russia and events elsewhere were to be judged by different standards. The endless executions in me purges of 1936-8 were applauded by life-long opponents of capital punishment, and it was considered equally proper to publicise famines when they happened in India and to conceal them when they happened in me Ukraine. And if this was true before the war, the intellectual atmosphere is certainly no better now.