The Soviet Government considers the recent behavior of the Polish Government with regard to the USSR as entirely abnormal, infringing all the rules and norms of mutual relations between two allied states.
Hostile to the Soviet Union, calumnious campaign launched by the German fascists in connection with the murder of the Polish officers, committed by themselves on the territory occupied by German armies in the Smolensk area, was taken up instantly by the Polish Government and is being fanned by every possible means by the Polish official press. The Polish Government not only failed to offer a rebuff to the vile fascist calumny, but it did not even find it necessary to address an inquiry to the Soviet Government, or request for an explanation on this issue.
The Hitlerite authorities, having committed a monstrous crime on Polish officers, are now staging a farcical investigation, in which they use Polish pro-fascist elements, whom they selected in Nazi-occupied Poland, where everybody is under Hitler's heel and where an honest Pole cannot openly have his say.
Both the Polish Government and the Hitlerite government drew the International Red Cross into the "investigation", making it compelled to participate in this farce of investigation in which Hitler appears to be the stage manager, in conditions of a terrorist regime with its gallows and mass extermination of the peaceful population. Understandably, such an "investigation", conducted behind the back of the Soviet Government, cannot command confidence of people, possessing any degree of honesty.
[...]
The Soviet Government is aware that this hostile campaign against the Soviet Union have been undertaken by the Polish Government in order to exert pressure upon the Soviet Government by making use of the calumnious Hitlerite fakes for the purpose of wresting territorial concessions from them, at the expense of the interests of the Soviet Ukraine, Soviet Byelorussia and Soviet Lithuania.
All these circumstances compel the Soviet Government to recognize that the present Polish Government, having slid on the path of collusion with the Hitlerite government, have actively severed allied relations with the USSR, and have stood in a position hostile to the Soviet Union.
On these grounds, the Soviet Government have decided to sever relations with the Polish Government.
Moscow, April 25, 1943
Stosunki Rzeczypospolitej Polskiej z państwem radzieckim 1918-1943. Wybór dokumentów (Poland's Relations with the Soviet State in 1918-1943. Selected Documents), ed. Jerzy Kumaniecki, Warsaw 1991