Labour has not said whether it obtained the document from Reddit, which revealed it had banned 61 accounts following the investigation
The leader of Britain's main opposition Labour party on Saturday dismissed as a "conspiracy theory" claims that a leaked classified government document used in his election campaign was put online by Russia.
Jeremy Corbyn has said the files containing details of trade talks between British and US representatives prove the government would "sell-out" the National Health Service (NHS) to US companies, a key campaign issue ahead of the December 12 vote.
The document first appeared on social network Reddit, which said on Saturday that it believed the leak "was part of a campaign that has been reported as originating from Russia."
Corbyn told Sky News on Saturday that the story of a Russia link was "such nonsense" and "an advanced stage of rather belated conspiracy theories by the prime minister."
"When we released the documents, at no stage did the prime minister or anybody deny that those documents were real," he added.
"Those documents show exactly what the British government was doing in discussions with Donald Trump's administration."
Culture Secretary Nicky Morgan told BBC Radio 4's Today program the allegations of interference were "extremely serious."
Labour has not said whether it obtained the document from Reddit, which revealed it had banned 61 accounts following the investigation.
Prime Minister Boris Johnson himself has come under scrutiny over why he has not released an official report into Russian interference in the Brexit referendum, and within his Tory Party.
"The Prime Minister has answers to give, which he refuses to do, about Russian donations to the Tory Party or the report that he is sitting on about Russian interference in British politics," said Corbyn.