Rose Cohen was born in 1894 in London, where her father Morris worked as a tailor after the family had fled persecution in Lodz, Poland. In those days, almost the only way for a working-class child from the East End to acquire an education beyond the three Rs was through the Workers’ Educational Association. Rose was a fast and hungry learner. The WEA gave her an extensive education in politics and economics as well as fluency in three languages. For a poor immigrant, and a woman at that, this was itself a remarkable achievement. It made her a welcome recruit, first on the staff of the London County Council, and then at the Labour Research Department, which had become a centre for young leftwing intellectuals by the end of the first world war. Rose, her friend and admirer Maurice Reckitt recalled, had great vivacity and charm. “It was natural,” he wrote, “that she should plunge into and swim with the communist tide which flowed so strongly in 1920.”

Indeed, Rose was a founder member of the British Communist party that year and, from the start, one of the small circle of young communists who soon came to dominate the party. She was, according to everyone who knew her, lively, intelligent, literate and hauntingly beautiful, with brown eyes and long dark hair. All the men who knew her talked of her smile, but say she was unaware of its magical quality. A packet of letters written to her during the 1920s clearly indicates a large band of obsessed male admirers throughout the British left.

Perhaps the most ardent of them all was Harry Pollitt. In 1926, the year of the general strike, Pollitt had already been identified by the Comintern as the man on whom the future of communism in Britain was going to depend. A short, heavily built boilermaker from Lancashire, Pollitt had a ready laugh and a natural warmth that communicated itself to those around him. A strong orator into the bargain, he quickly established himself as one of the key figures in the fledgling Communist party.

He fell passionately in love with Rose Cohen. On the back of a picture of Rose taken in Moscow is an inscription in Pollitt’s hand: “Rose Cohen, who I am in love with, and who has rejected me 14 times.” By 1925, he knew for certain he had lost her – to Max Petrovsky, whom Rose met in 1921 when the Soviets stepped in to guarantee contract work for the LRD to keep it alive after its links with the Labour party were severed.

Born in 1883 (or thereabouts – so much of his life was lived in secrecy that we cannot be sure of the exact date) in Ukraine, Max was the son of a wealthy merchant. His real name was David Lipetz, and at various times he used the names Petrovsky and Goldfarb. Leading Bolsheviks before the 1917 revolution generally had several names: it was how they stayed alive.

In 1924, Petrovsky was sent to Britain as the Comintern representative, to be guide and mentor to the infant British party. Petrovsky, when in England, used the name Bennett. He managed to avoid the British police for five years – a remarkable feat, which no subsequent Comintern representative ever equalled. His influence on the British Communist party was huge. It is unlikely that any major decision was made without his approval.

Life must have seemed like a wonderful adventure to the clever and beautiful young Rose Cohen in the 1920s. As a Comintern agent, she travelled the world, entrusted with secret missions and conveying not just messages but money and advice to communist parties everywhere. In 1927, Rose and Max Petrovsky moved permanently to Moscow. Whether they were married in London first, or in Moscow, we do not know for certain.

They were the golden couple of the expatriate community in Moscow: both had exciting, important jobs, and their son Alyosha was born in December 1929. They were devoted to him. They were sure not only of their own future, but of the future of the great socialist revolution of which they felt privileged to be a part. Rose seems to have deceived herself into believing that the average Muscovite benefited as much as she and Max did from the revolution. She did not see the germs of the great terror that was to come, though they lived in Moscow at the time when Stalin was tightening his grip.

In 1932, Rose became foreign editor of the new English-language paper, the Moscow Daily News. With her heavy duties, and Petrovsky’s senior job, they deposited Alyosha at their dacha outside Moscow in the care of a nurse, and stayed in their flat in Moscow, getting out as often as they could to see their son. As late as 1934, Rose was still writing transparently happy letters to her sister, Nellie Rathbone, in London.

Ivy Litvinov, the English wife of Maxim Litvinov, Stalin’s foreign commissar, knew Rose well and liked her, but perhaps saw more clearly than Rose the way things were going. When the terror started, Ivy wrote many years later, Rose “was always rather smug – ‘Well, I suppose they know what they’re doing, don’t you?’ ”

The big show trials began in August 1936. The first famous and powerful Bolsheviks to go to the firing squad were the same two men whom foreign communists had been taught to regard as heroes of the revolution: Kamenev and Zinoviev. The Comintern laid down the correct line for the British Communist party to take: that the two men were “mean degenerates” and “abominable traitors” who were in league with Trotsky and Hitler’s Gestapo and had plotted “huge crimes”.

Kamenev and Zinoviev were shot. All the most distinguished of the old Bolsheviks followed them to execution. No one knows exactly how many people were murdered over the next three years as the Communist party and the Soviet Union were “purged”, but it runs into millions. Robert Conquest says there were six million arrests, three million executions and two million deaths in the camps. Subsequent writers have put the figures rather lower, but the numbers are still huge.

“Do please write soon. M is away and I’m feeling very lonely,” wrote Rose from Moscow to Nellie in London in April 1937. “M is away.” In a letter that she knew would be opened by the authorities, Rose Cohen could not tell her sister the appalling truth. A few days earlier, on March 11, the secret police had come for Max. A month later, the British Communist party’s political bureau was told that he had been arrested as a “wrecker” and all British communists who had had any contact with him were to make statements giving full details of what they knew about him.

Harry Pollitt, who did know about Max’s arrest, wrote to Rose. It was an attempt to cheer her up: “My visit to Spain [to the International Brigades fighting in the Spanish civil war] gave me great satisfaction. There is quite a story about how I got there, which will make you laugh …” And then, at the end, he wrote as much as he dared about her troubles: “We all send our love. Don’t lose heart.”

She never saw Harry’s letter. I found it, more than half a century later, in a dusty Comintern archive, where it was originally deposited, so that it could be used, if need be, to help show that Pollitt, too, was a traitor.

Ivy Litvinov’s frightened husband had said to her: “I hope you’re not going to rush off and see Rose. It would be a terrible thing for you to do.” But Ivy’s daughter Tanya went to Rose and brought her to Ivy. “Not a single one of my friends has been to see me,” said Rose to Ivy, who recalled later: “She was utterly lonely and trembling.”

According to Ivy, Rose’s greatest fear was that she would be arrested. What, then, would happen to eight-year-old Alyosha? She knew something of the dreadful orphanages to which the children of “enemies of the people” were sent, where they were treated harshly and forbidden ever to mention the names of their parents.

On July 4, Rose wrote to her sister again: “Thanks for remembering my birthday and your greetings. They were more than welcome, for no one else remembered except my friend Masha in New York, who never forgets … I’m sorry this is such a dull letter, but I’m feeling in a dull mood. So I had better stop … Do please write oftener.”

Nellie could not know what was wrong, but she caught the mood and replied at once. Her letter was returned. The knock on the door at Rose’s Moscow flat came at 3am on August 13 1937.

That same day, Pollitt arrived in Moscow to discuss the political situation in Britain. This would have been his first meeting with Rose after Petrovsky’s arrest. Presumably the secret police did not want this meeting to take place. They may also have hoped that they could squeeze out of Rose something they could use against Pollitt. At all events, they kept Pollitt kicking his heels for days in the Hotel Lux while they questioned Rose Cohen.

Just what was going on in the minds of Pollitt and his colleagues, who had regarded Max and Rose as close friends? Pollitt wrote to the general secretary of the Comintern, Georgi Dimitrov, of his “very warm personal friendship” with Petrovsky and his confidence in his unwavering loyalty to Stalin and the international communist movement. The news of the arrest, he wrote, came as “one of the greatest shocks of my life”.

But in public, Britain’s communist leaders threw their old friends to the wolves. When Rose’s arrest became known in England, which was not until months after it happened, and other newspapers demanded that the Soviets at least charge her publicly and say where she was, the Daily Worker snarled at their anti-Soviet stance, and furiously insisted that the British government had no right to intervene. It was a weaselly way to abandon a woman whom every leading communist in Britain had counted as a friend – somehow even more distasteful because the editorial fastidiously declined to name her.

William Gallacher, Britain’s only communist MP, went to see Dimitrov. When Gallacher raised the question of Rose, Dimitrov looked at him gravely for a few moments, and said: “Comrade Gallacher, it is best that you do not pursue these matters.” And Gallacher did not.

Privately, Pollitt was distraught, and pressed his protests about Rose Cohen further than his colleagues thought wise. He had a long interview with Dimitrov, and according to Ivy Litvinov, he even saw Stalin himself, and asked him to send Rose back to England.

Comintern officials started to suggest to other leading British communists that Pollitt was not as reliable as he ought to be, and they should think about replacing him. Things were worse for Pollitt than either he or they knew. Rose was one of several foreign communists and Comintern leaders who were arrested in the spring and summer of 1937. People will say anything under torture. We know that the Hungarian Bela Kun named Pollitt as one of his “collaborators” in his Trotskyist “conspiracy”, and he may not have been the only one.

Whatever Dimitrov said, Pollitt and his colleagues not only ensured that the British Communist party failed to protest, but also prevented effective public protests from anyone else on the left. Approaching public figures on the left, Rose’s old friend Maurice Reckitt found it hard to get significant leftwingers to sign a very moderate letter to the New Statesman protesting at her disappearance.

If British communist leaders behaved discreditably in abandoning Rose, so too did the British government. The Foreign Office seems to have been eager to seize on the excuse that Rose had given up her British citizenship and become a Soviet citizen, in order to wash its hands of her. In fact, my reading of British Foreign Office documents now available suggests that it is most unlikely she did anything of the kind. She was a British citizen who had never given up her citizenship. The Soviet Union persecuted her falsely for being a British spy. The British government failed her because she was a communist.

The official who briefed the foreign secretary commented smugly that the British Communist party seemed to carry little weight in Moscow: “The history of the Russian revolution shows that the ‘Bloomsbury Bolshevik’ or ‘parlour pink’ is the first to go to the wall.” The Daily Sketch gloated: “A woman who was an apostle of this anti-democratic creed must rely now upon the protests of a ‘capitalist’ state.” It was not much to rely on.

Rose, it is said, broke down completely in prison, and cried constantly for Alyosha, right up to the moment, on November 28, 1937, when they took her out and shot her. Petrovsky was already dead: he had been shot in September.

Stalin’s British Victims by Francis Beckett is published by Sutton Publishing, price £20. To order a copy for £17 plus p&p, call the Guardian Book Service on 0870-836 0875.