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".