Leaked classified documents detailing plans for a post-Brexit US trade deal – which Jeremy Corbyn claimed showed a Tory plot to “sell off the NHS” – may have been posted online by a Russian campaign, it has emerged.

The 451-page dossier, which the Labour leader unveiled during a press conference on Wednesday, is believed to have been posted to a politics forum on social media website Reddit back by a user named “gregoratior” back in October.

A social media research firm earlier this week said that the documents were shared in a similar way to a Russian disinformation operation dubbed as Secondary Infektion, which was exposed by Facebook earlier this year.

And Reddit has now confirmed that they believe the documents did come from Russia.

The company said in a statement: “We were recently made aware of a post on Reddit that included leaked documents from the UK.

Top 10 marginal seats to watch at the general election Show all 10 1 /10 Top 10 marginal seats to watch at the general election Top 10 marginal seats to watch at the general election North East Fife Currently held by the SNP with a majority of 2 Getty Top 10 marginal seats to watch at the general election Kensington Currently held by Labour with a majority of 20 Getty Top 10 marginal seats to watch at the general election Perth and North Perthshire Currently held by the SNP with a majority of 21 Getty Top 10 marginal seats to watch at the general election Dudley North Currently held by independent, formerly Labour, MP Ian Austin with a majority of 22 LivingInMediocrity Top 10 marginal seats to watch at the general election Newcastle-under-Lyme Currently held by Labour with a majority of 30 Derek Harper Top 10 marginal seats to watch at the general election Southampton Itchen Currently held by the Conservatives with a majority of 31 Rob Candish Top 10 marginal seats to watch at the general election Richmond Park Currently held by the Conservatives with a majority of 45 Robin Webster Top 10 marginal seats to watch at the general election Crewe and Nantwich Currently held by Labour with a majority of 48 Jaggery Top 10 marginal seats to watch at the general election Glasgow South West Currently held by the SNP with a majority of 60 Alec MacKinnon Top 10 marginal seats to watch at the general election Glasgow East Currently held by the SNP with a majority of 75 Christine Johnstone

“We investigated this account and the accounts connected to it, and today we believe this was part of a campaign that has been reported as originating from Russia.”

The company said it has also identified a further network of accounts, which had attempted to promote the original post with the leaked documents.

“All of these accounts have the same shared pattern as the original Secondary Infektion group detected, causing us to believe that this was indeed tied to the original group,” it said.

Mr Corbyn claimed the documents contradicted the prime minister’s denial that the NHS is part of negotiations with the US, which some fear could see parts of the public health service sold off to private US companies.

He told reporters: “We have now got evidence that under Boris Johnson the NHS is on the table and will be up for sale. He tried to cover it up in a secret agenda and today it has been exposed.”

A Labour spokesperson declined to comment on how the party obtained the leaked documents or ties to the Russian campaign.

The Reddit post, titled “Official-sensitive: Great Britain is practically standing on her knees working on a trade agreement with the US”, were only seen by a handful of users before Mr Corbyn came forward with them.

“I suspect that this publication will make some noise, so that’s why you probably don’t have much time to look through the internal secret documents that contain specific details of the upcoming FTA between the UK and the US,” the Reddit user wrote.