Fifty years ago, on Christmas Day 1970, a handful of British Jews gathered in the bitter cold outside the Soviet Embassy in Bayswater. The news had reached London the night before that two Soviet Jews, Mark Dymshits and Edward Kuznetsov, had been sentenced to death. The announcement on Christmas Eve of these draconian sentences had been deliberate — in those days there was no 24-hour rolling news coverage, no internet, no Twitter or Facebook. The KGB had counted on festive indifference and a silent night in the media.

The other defendants were given long terms in strict regime prison camps. All had been involved in an attempt to take a 12-seater aircraft at Leningrad’s Smolney airport, fly it to Priozersk near the Finnish border, pick up another four passengers — and hop over the border to Sweden.

All had wished to leave for Israel, been repeatedly denied visas and could bear the waiting no longer. Their frustration had given way to a fatalism that anything was better than endlessly sitting on suitcases. Dymshits, the pilot, had commented that they had a 5 per cent chance of success — and remaining alive.

Bizarre schemes of escape had proliferated — a hot air balloon, seizing a submarine, swimming to Turkey. The idea of flying to freedom was not new. In 1945, Jews boarded small planes from Lithuania en route for the Promised Land — or so they thought. The pilot took off and then returned to the airport. The police eagerly awaited the passengers before dispatching them to the Gulag. This ruse was carried out six times.

After the Six-Day war, a growing number of Jews wanted to leave for Israel. They had grown tired of the hollow slogans of the Kremlin and gradually reclaimed their Jewishness from the official policy of assimilation. Many of the defendants had come from Riga in Latvia which had only recently become part of the Soviet Union in 1944. Iosif Mendelevich, from a traditional family in Riga, wore a kippah during the trial.

Material about Zionism and Israel had reached them. This included Leon Uris’s Exodus, writings by the Israeli satirist Ephraim Kishon, Hebrew poems by Bialik and even a profile of the new Israeli prime minister, Golda Meir, from The Times.

Following Stalin’s death in 1953, his successors resolved not to continue his lethal policy of murderously eliminating any perceived threat. The gates of the Gulag were opened and prisoners who had spent decades in camps returned. This thaw produced Zionists as well as Jewish dissidents who wished to change the USSR for the better.

Israelis were allowed to attend the International Youth Festival in Moscow in 1957 — and many young Jews in the USSR flocked to meet them.

Even so, during the 1950s, now forgotten figures such as David Khavkin, Tina Brodetskaya and the Podolsky family, were sentenced to long years in camps for their desire to leave for Israel. The KGB sent many of these early Soviet Zionists to the same camps. They met, shared knowledge and thereby cemented a network of Zionist Jews who had had no previous contact. Ironically it was the KGB which laid the foundations for the Jewish emigration movement in the USSR.

The head of the KGB, Yuri Andropov, had a track record of cracking down on dissidents and persecuting writers such as Aleksandr Solzhenitsyn. He also promoted the use of imprisoning critics in psychiatric hospitals. In April 1970, he reported to the central committee of the Communist party that the Leningrad Zionists were developing a secret operation with Riga Jewish nationalists.

The KGB knew everything from an early stage and planned to entrap the entire leadership of the Jewish movement in every Soviet city. Within minutes of the arrests on the tarmac, the homes of activists were being searched, material confiscated and arrests made. The irony was that many believed that the aeroplane plan had been dropped after strong opposition from mainstream activists and from circles in Israel. Even Kuznetsov suggested a postponement for a year. The driving force in continuing was Dymshits’s “now or never” attitude — and the others went along in a fatalistic fashion, knowing that it was doomed to failure.

Kuznetsov believed that the Soviet Union was collapsing under the weight of its own economic system. The failure of an amateurish attempt to take an aircraft would be a wakeup call to the diaspora to mobilise for the cause of Soviet Jewry. He further argued that détente with the United States was on the horizon and therefore Soviet Jews who wished to leave would have leverage. No one, however, expected charges of high treason and the death sentences.

Dymshits had lost both his parents in the Nazi siege of Leningrad, suffered from antisemitism and was halted in advancing his career as a pilot. Kuznetsov’s mother had changed the family name of Gerson in 1953 — the year of the Doctors’ Plot when mainly Jewish physicians were accused of conspiring to poison the leaders of the USSR.

Kuznetsov had come of age in the 1950s and been involved with dissident literary journals such as Syntax and Phoenix. He was arrested and sentenced to seven years in a camp. On release, he gradually became aware of his Jewishness and concluded that “the political tradition of the Russian people is one of despotism”. He met and married Sylva Zalmanson and soon became a seminal figure in the emigration movement in Riga.

After the sentences were announced on Christmas eve, Jewish activists made certain that an accurate version of events was conveyed to London. They were helped by sympathetic non-Jewish dissidents such as Vladimir Bukovsky, Vladimir Telnikov and Valery Chalidze. All were shocked by the spontaneous applause of the KGB-selected audience in the courtroom when the death sentences were passed.

A wave of protests was ignited — from the Pope to the British Communist party. The French Communist daily, L’Humanité and its Italian equivalent, L’Unita, were highly critical. In Genoa, dockers refused to load Soviet ships. In Chile, the newly elected Marxist president, Salvador Allende, called for commuting the sentences.

The Kremlin was overwhelmed by the worldwide disgust at the verdict. Devout Christians remembered that a Jew, Jesus Christ, had been born at Christmas and had died for his beliefs. All this stimulated an intense discussion within a startled Soviet leadership. But perhaps a deciding factor was the decision of the Spanish government to annul death sentences passed on a group of Basques at Burgos. How could the heirs of Lenin fall below the standards of the fascist dictator, Francisco Franco?

Several years ago, it was revealed that Franco’s move had come about because of a private appeal from Golda Meir. This, in itself, was remarkable because Israel had refused to establish diplomatic relations with Spain. Golda Meir remembered Franco’s friendship with Hitler and the antisemitic utterings of many of his generals. Many British Jews refused to visit Spain while Franco was alive.

Franco survived because the Americans needed him as an ally during the Cold War. Helping Dymshits and Kuznetsov was part of his rehabilitation.

Kuznetsov records in his diary that he was summoned by Major Kruglov and told that “as a humanitarian gesture, the sentence of death passed against you has been commuted to 15 years on special regime. May I wish you a Happy New Year.”

Kuznetsov was bemused at this greeting “from this blue-epauletted Santa Claus”.

Dymshits and Kuznetsov secured an early release in 1979 in exchange for Soviet intelligence officers in US prisons. Dymshits died in Israel five years ago while Kuznetsov, now in his eighties, lives in Jerusalem.

The aeroplane affair awoke British Jews from their slumber to campaign for Soviet Jewry during the 1970s. For this generation who grew up in the shadow of the Shoah and bore witness to the rise of Israel, “Never Again!” became more than a slogan.

Colin Shindler was active in the campaign for Soviet Jewry in this country between 1966 and 1975