Here's the evidence. It's the official diary -- that is, report -- by Potemkin, the Vice-Commissar for Foreign Affairs of the USSR, about his visit with Grzybowski, Polish Ambassador to the USSR, at 3 a.m. on September 17, 1939.
Potemkin wishes to hand a note to the Polish Government. At first Grzybowski refuses to accept it. But when pressed, Grzybowski admits that he has no contact with his government. Grzybowski says that two days before he had been told to contact his government "via Bucharest" -- i.e. in the Rumanian capital -- but he is not certain that he could do even that.
I have put Ambassador Grzybowski's "note", with accompanying textual criticism, online here, with the Russian version here.
I have put an English translation of Potemkin's note, followed by the Russian original, on line here. The original source is: Äîêóìåíòû âíåøíåé ïîëèòèêè ÑÑÑÐ. Òîì 22. Êí. 2. 1 ñåíòÿáðÿ — 31 äåêàáðÿ 1939 ã. Ì.: Ìåæäóíàðîäíûå îòíîøåíèÿ, 1992, Document No. 596, pp. 94-95. I have taken the Russian text from this source.
The Russian original is also published in the book Katyn' -- Plenniki Neob"iavlennoi Voiny on pp. 65-67. This is a ferociously anti-Soviet, Polish nationalist work. (http://katynbooks.narod.ru/prisoners/Docs/007.html ) This edition has a somewhat different header, which I can't explain. There is an English translation in in Anna M. Ciencala, N.S. Lebedeva, Wojciech Materski, eds., Katyn. A Crime Without Punishment. New Haven: Yale University Press, 2008, pp. 45-47.
The note itself, signed by Molotov, People's Commissar for Foreign Affairs, is reproduced many places. I've prepared a page in English and one in the original Russian with these documents.
The original source is ibid. Document No. 597, p. 96.
Here is a part of Molotov's statement about the reasons for Soviet intervention as published in the New York Times of September 18, 1939, p.5:
|Warsaw as the capital of the Polish state no longer exits. No one knows the
whereabouts of the Polish Government.
The population of Poland have been abandoned by their ill-starred leaders to their fate. The Polish state and its government have virtually ceased to exist.
In view of this state of affairs, treaties concluded between the Soviet Union and Poland have ceased to operate.
… Poland has become a fertile field for any accidental and unexpected contingency that may create a menace to the Soviet Union.
The same NYT article reprinted a part of Molotov's statement to the Polish ambassador Grzybowski - the one cited above -- which Grzybowski refused to accept, while agreeing that he did not know where his government was:
|Warsaw as the capital of the Polish state no longer exits. The Polish Government has
fallen to pieces and shows no signs of life.
This means that the Polish State and its government have virtually ceased to exist. Treaties concluded between the USSR and Poland have ceased to operate.
Abandoned to her fate and left without leadership, Poland has become a fertile field for any accidental and unexpected contingency that may create a menace to the USSR.
New York Times reporter Jerzy Szapiro observed in an article published on October 2, 1939, p. 8: “Polish Government Now Blamed For Nation’s Military Collapse. Failure of Moscicki Regime to Remain in Warsaw Seen as Vital Factor.”
|The government is accused of losing its nerve when, on the fifth day of the war, the signal was given for the flight from Warsaw. Polish refugees of all political opinions, even supporters of the regime, are now convinced that had the government remained and had the highest army leaders stayed at their posts Russia might not have marched and certainly would not have the formal excuse of advancing into a country abandoned by its government.|
As Szapiro pointed out, the date of September 5, when the Polish government fled the Polish capital and after which it never set up a new capital, was a plausible date after which one could consider that Poland had no government and consequently had ceased to exist as a state.
But Szapiro is wrong in calling this a “formal excuse.” It’s a fundamental issue. No government means, among other things, no one to negotiate with.