The article is an attempt to observe the evolution of the role of the Soviet factor in British-Czechoslovak relations during the Second World War. In the months  preceding and at the beginning of the war, its influence was barely noticeable. The USSR then acted as an ally of Germany. Only in August 1940 did the FO note attempts to establish cooperation between the Soviet government and the Czechoslovak Provisional Government. From the fall of 1940, contacts were developed between the Soviet and Czechoslovak intelligence services. The role of the Soviet factor in Czechoslovak policy began to grow rapidly from the summer of 1941 – the entry of the USSR into the war with Germany and Moscow's full recognition of the Czechoslovak government in exile. The USSR's position on this matter forced Great Britain to similarly recognize the Czechoslovak authorities. Since then on, the Soviet factor as a lever for achieving political goals in relations with the British was used permanently and on an increasing scale by Czechoslovak diplomacy. Moscow's support (this time ineffectively) was also used to force the British to recognize the pre-Munich borders of the ČSR and the so-called ʺRevocation of Munichʺ – thus recognizing the invalidity and illegality of the Munich Agreements of 1938 from the very beginning of their existence. London observed with concern the decline of Czechoslovak diplomacy into the position of a Soviet vassal, especially clearly visible in the  forced abandonment of its plans for federation with Poland demanding by Kremlin. From these positions, the FO opposed Beneš's visit to Moscow, which was expected already in April 1943 and which threatened to deepen Poland's isolation after the Soviet authorities broke off relations with it. Beneš tried to discredit the opinions about the Soviet invader policy and eventually paid a visit to Moscow and led to the signing of the Czechoslovak-Soviet alliance agreement, but only in December 1943. From that moment on, ČSR was perceived on the Thames as a country in the Soviet sphere of influence and the structures of the Czechoslovak authorities in exile were considered to be infiltrated by communists – and therefore by Moscow. When withdrawing its opposition to the Czechoslovak-Soviet Treaty, the British government simply drew pragmatic conclusions from the fact that the Red Army, as an ally in the war with Germany, was a fundamental factor in bringing about the defeat of the Third Reich and as such was needed by London, and from the belief that then the Soviets will occupy the Czechoslovak lands and in any case they will have a huge influence on the decision regarding them. This belief also largely determined the British activity towards the uprising in Slovakia in 1944 and Prague in 1945. It was considered that this was a ­Soviet zone of military responsibility and only occasionally any military activity was undertaken there, encountering reluctance from the Soviet side. The title of a voluntary vassal of the USSR permanently stuck to the Czechoslovak government in exile. This situation strengthened the FO's tendency to reduce interest in Czechoslovak affairs. Beneš's capitulation to the occupation and annexation of Transcarpathian Ruthenia to the USSR confirmed, in the eyes of the FO, the thesis that the Czechoslovak authorities were subordinated to Stalin's orders. This became fully visible after the ČSR authorities returned to the country via Moscow, where the government was reconstructed, giving most of the influence to the communists. Attempts to persuade the Americans to outdo Soviet troops in taking Prague, as well as hopes of maintaining British influence in post-war Czechoslovakia, turned out to be in vain.