General election 2019: Source of UK-US trade document leak must be found - PM
Boris Johnson has said an investigation is needed into the source of leaked documents on UK-US trade negotiations posted on Reddit.
Labour says the documents show the NHS would be at risk under a post-Brexit trade deal with the US.
On Friday, forum website Reddit said unredacted documents were uploaded as "part of a campaign that has been reported as originating from Russia".
It has suspended 61 accounts that showed a "pattern of coordination".
The government said it was looking into the matter with help from the National Cyber Security Centre.
Speaking on Saturday, Mr Johnson said "we do need to get to the bottom" of the leak but said he had seen "no evidence of any successful interference by Russia in any democratic event in this country".
The Culture Secretary Nicky Morgan said this all pointed towards foreign involvement: "I understand from what was being put on that website, those who seem to know about these things say that it seems to have all the hallmarks of some form of interference."
Labour's shadow transport secretary Andy McDonald reiterated his call for Mr Johnson to release an intelligence report into Russian covert actions in the UK, which No 10 has been accused of suppressing until after the election.
In a post on its site, Reddit did not provide any further details about the evidence behind its conclusions, nor did it identify any specific individuals.
The BBC has approached the Russian foreign ministry spokesperson but they have yet to comment.
The contents of the documents have played a significant part in Labour's election message on the NHS, after Mr Corbyn highlighted them at a press conference on 27 November.
The Labour leader said the papers were evidence that the UK government was in advanced stages of negotiations with the US to open up the NHS to American pharmaceutical companies.
Labour have not said where they obtained their copy of the documents.
A version of the documents, heavily redacted, was also produced by Mr Corbyn at an earlier leader debate on 19 November.
At the time, Labour said these were the result of a Freedom of Information request by campaign group Global Justice Now.
The dossier was posted on Reddit more than a month prior to Mr Corbyn's announcement, prompting questions about how they got there - and why few people seemed to notice them before.
'Double-edged sword for Labour'
Analysis by political correspondent Jonathan Blake
A bit like journalists never reveal their sources, Labour are quite happy to focus on what these documents say rather than where they come from.
If you look at where Reddit's comments leave the discussion, it's both helpful and slightly problematic for Labour.
On the one hand, people are asking "where exactly did you get those documents from?" Remember, they were online in their unredacted form for several weeks before Labour brought them to everyone's attention.
But at the same time, we're still talking about these documents and what Labour claims that they show - that the NHS is up for sale, in their words. Boris Johnson and the Conservatives flatly deny that.
So it's a double-edged sword for Labour.
For the Conservatives, you've got this uneasiness around Russian interference in an election campaign - which isn't good for them because attention will turn to the report by Parliament which the government hasn't released.
And that's not very helpful for the Tories either.
Speaking on Saturday, the Labour leader said the controversy surrounding the source of the documents was "nonsense" and accused Mr Johnson of wanting to "hide the issues and the truth" over the future of the NHS in trade deals.
Mr Johnson said the documents "didn't prove what Jeremy Corbyn and the Labour party hoped it would prove" adding "it was just another distraction from the void at the heart of Labour's policy on Brexit".
Neither UK nor US governments have disputed the authenticity of the documents.
The BBC's security correspondent Gordon Correra said crucial questions remained as to how the document circulating online originally appeared.
He said there would be a significant difference between a state-led operation from Moscow which hacked the material and then leaked it as opposed to someone who is based in Russia simply opportunistically using an already leaked document to cause mischief.
"That question is one that national security officials will be trying to answer."