The purges of the 1920s and 30s had destroyed the writers: Mandelstam, Babel, Yesenin, Mayakovsky, Tsvetaeva, Gumilyov and many others were dead, executed or by their own hand. Then, in 1948, Stalin turned to the composers. The Great Leader and Teacher had heard an opera that displeased him. His anger spread to all avant-garde music, to all music that didn't fit his own taste for old-fashioned, accessible melodies, easily understood by the people, upbeat and celebrating the superiority of all things Soviet. Stalin ordered his commissars to impose socialist realism in music, and to weed out those who had other ideas. The Central Committee drew up a decree condemning composers of music that was "inimical to the people" and "formalist".

They handed the task of wiping out formalism to the head of the soviet composers union, Tikhon Khrennikov. At the first congress of the union of composers from April 19-25 1948, Khrennikov listed those who were in the firing line: the "elitist, anti-socialist" Shostakovich, Prokofiev, Khachaturian, Miaskovsky and others ... "In the music of Comrade Shostakovich we find all sorts of things alien to realistic Soviet art, such as tenseness, neuroticism, escapism and repulsive pathology. In the work of Comrade Prokofiev ... natural emotion and melody has been replaced by grunting and scraping."

Khrennikov reported that people "all over the USSR" had "voted unanimously" to condemn the so-called formalists and let it be known that those named in the decree were now officially regarded as little better than traitors: "Enough of these pseudo-philosophic symphonies! Armed with clear party directives, we will stop all manifestations of formalism and decadence."

For Shostakovich, undoubtedly the main target and whose satirical operas and ballets are being performed by Valery Gergiev and the Mariinsky Theatre at the London Coliseum this month, it was a terrifying moment. The guilty men were forced into a public recantation of their errors and a humiliating exhibition of self-criticism and abasement. Prokofiev suffered a stroke and never recovered; he died five years later, on the same day as Stalin in 1953.

Shostakovich - who had been through the same public flagellation 12 years earlier - knew he must bite his tongue and confess: "I thank you comrade chairman ... I thought I had succeeded in developing a personal idiom that adhered to the wise demands of the Soviet people ... I now see I was mistaken and have underestimated my need for artistic correction. I acknowledge the rightness of the party's judgment. I shall work on the musical depiction of the heroic Soviet peoples, from the correct ideological standpoint. Equipped with the guidance of the Central Committee, I shall renew my efforts to create really good songs for collective singing."

Shostakovich's recantation is so abject and so exaggerated that it is tempting to conclude he was mocking. In fact, he was left terrified and crushed by the ordeal. In private, though, he was preparing his revenge.

All those attacked by Khrennikov in 1948 knew their careers were stymied, and until Stalin's death they lived in constant expectation of arrest, imprisonment or even execution. Astoundingly, Khrennikov remained in his post as chief arbiter and inquisitor of Russian musical life until 1991. He is now aged 93 and agreed to talk to me in Moscow last month.

When I suggest he led the regime's repression of musical life, he becomes angry and yells at me that I am recounting lies and slander; he says the reason the Soviet Union needed to encourage positive socialist realism in music was because "you" (the west) had erected an iron curtain to threaten the USSR; the campaign against Jewish composers was regrettable, he says, "but don't forget there were many Jews in musical life and they launched unfair attacks on my compositions".

Khrennikov tells me he was simply told - forced - to read out the speech attacking Shostakovich and Prokofiev in 1948: "What else could I have done? If I'd refused, it could have been curtains ... death. They made me do it; and anyway, Shostakovich and Prokofiev were sympathetic to my plight - they knew I had no choice: I did everything I could to help them financially while they were banned and repressed ... and they were grateful to me".

But even now he is proud of the power he wielded under Stalin: "My word was law", he says. "People knew I was appointed personally by Stalin and they were afraid that ... I would go and tell Stalin about them. I was Stalin's Commissar. When I said No! (he shouts), it meant No."

Khrennikov tells me with relish of his own meetings with Stalin: he was a connoisseur of art and music; he understood it much better than anyone, so much so that he would hold Politburo meetings in a private box at the Bolshoi: when the most accomplished singers came on stage, Stalin would hold up his hand and order a pause from the mighty affairs of state to hear the voice of genius.

And Khrennikov was brave, he says; he would argue with the Great Leader over the fate of musicians, defending the truly deserving among them and condemning those who fell short. But that is not how it seems to Shostakovich's widow, Irina. She shows me into the Moscow apartment where they lived together and points to the drawer where her husband consigned the many compositions Khrennikov and his lackeys had banned. She says that Shostakovich's self-abasement and "recantation" after the attacks from Khrennikov were, of course, feigned: her husband never accepted the criticisms that were levelled at him and never recognised the authority of those who made them.

If you want to know what he really thought, she says, you need to listen to a piece of bitter musical satire he composed after Khrennikov's lambasting and had to keep hidden for many years while the intimidation continued. The piece - entitled "The Anti-Formalist Gallery" (Antiformalisticheskiy Rayok) - was not performed until long after Shostakovich's death. In Moscow last month, I attended a rare staging of the work. A bass dressed as Stalin and singing in his distorted, unmistakable, Georgian accent, belted out a deliberately ungainly and hilarious aria with words taken directly from the speeches denouncing 'formalism' at the 1948 congress, emphasising the ungrammatical, ignorant style of official party-speak:

"Dear Comrades, while realistic music is written by the People's composers, formalistic music is written by composers who are against the People. Comrades, one must ask why it is that realistic music is always written by composers of the People? The People's composers write realistic music simply due to the fact that being by nature realists right to their very core, they simply cannot help writing music that is realistic, while those anti-People composers, being by nature unrepentant formalists, cannot help ... cannot help ... cannot help writing music that is formalistic ..."

"The Poet", a character dressed to look like the famous actor Smoktunovsky playing Hamlet in Pasternak's sublime translation, poked his head nervously from the wings and sang, with obvious distaste:

"Thank you comrade for that lucid and highly informed oration that did so much to educate us and elucidate such vital questions on the subject of music."

Anyone who still entertains the misconceived idea that Shostakovich was a cringing lackey of the state and sycophant to Stalin could do worse than listen to his Anti-Formalist Gallery. It is as hard hitting and caustic as Mandelstam's famous poem mocking Stalin that got him sent to the gulag and an agonised death in Siberia. Shostakovich just had the good sense to keep his bilious mockery "in the drawer".

Shostakovich suffered as a result of the 1948 decree, but he went on to write the greatest of his mature works in the quarter century that followed, including the last six symphonies, the last 10 quartets and the magnificent preludes and fugues. At worst, one might regret that the official demolition of his Lady Macbeth of Mtsensk in 1936 scared him off any further opera writing (interestingly, Gergiev and the Mariinsky are doing Shostakovich's "toned down" version of the opera, rechristened Katerina Izmaylova, which he wrote to get the work performed during the Khrushchev and Brezhnev years). Later, he told Flora Litvinova, a friend and writer, "without 'party guidance' ... I would have displayed more brilliance, used more sarcasm; I could have revealed my ideas openly instead of having to resort to camouflage".

Prokofiev, it is true, was left a broken man, but his relations with the regime were already difficult before 1948, his private life was complex (he had actually left his wife Lina before Stalin sent her to the camps) and who is to apportion blame for that? Who is to decide what caused his early death? Whatever else he did, it is undeniable that Tikhon Khrennikov - possibly under pressure from his conscience and certainly under pressure from Mstislav Rostropovich - sent Prokofiev financial help while he was in official disgrace.

Khrennikov was certainly flattered by the power and influence Stalin conferred on him, and he did his master's bidding with a vengeance: his ruthless imposition of "socialist realism" dogged Soviet music for decades and tormented the greats like Shostakovich and Prokofiev. But he is right when he says it would have been someone else if he had turned it down. And it is undoubtedly true that composers and musicians avoided the mass arrests and executions that Stalin inflicted on the writers.

· Shostakovich on Stage, a festival of the composer's satirical operas and ballets performed by Valery Gergiev and the Mariinsky Theatre, is at the London Coliseum, from July 20-29. Box office: 0870 145 0200. Martin Sixsmith's new novel, I Heard Lenin Laugh, is published by Macmillan. His Radio Four series on Russian literature and music, "Challenging the Silence", will be broadcast on July 31, August 7, 14