British Labour leader Jeremy Corbyn has dismissed as "nonsense" the controversy surrounding the source of leaked documents used by his party as proof the Conservatives are planning to sell the NHS.

He insisted the dossier was real after social media platform Reddit said the documents were disclosed as "part of a campaign that has been reported as originating from Russia".

Mr Corbyn used the leaked papers as evidence that the government was at an advanced stage of negotiations with the US to open up the health service to US pharmaceutical companies.

In a statement, Reddit said its investigation had found a "pattern of co-ordination" between the now-banned accounts on its site and a Russian campaign uncovered by Facebook earlier this year.

Information from the investigation into the "Secondary Infektion" campaign on Facebook was used to identify suspect accounts on Reddit.

Mr Corbyn was asked about the claims during a campaign visit to Barry, South Wales.

He told reporters: "This is such nonsense. This is such 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, deny the arguments that we put forward, and if there has been no discussion with the USA about access to our health markets, if all that is wrong, how come after a week they still haven't said that?

"The issues are that those documents show exactly what the British government was doing in discussions with Donald Trump's administration in the USA and also why the Prime Minister has refused to release the report on Russian interference in British politics, which he's been sitting on for a very long time."

He accused Boris Johnson of wanting to "hide the issues and the truth" over the future of the NHS in trade deals, adding: "We obtained those documents, we believe those documents to be correct, and nobody until yesterday had denied the correctness of those documents.

"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.

"Of course there should be no interference in our British political system by Donald Trump or the Russians."

Speaking at a news conference last month, Mr Corbyn said the 451 pages left Mr Johnson's claim that the NHS would not be part of trade talks "in absolute tatters".

Reddit said the leaked files had first been posted in October and then reposted by a second account.

Pockets of accounts had worked together to "upvote" the posts using vote manipulation to make them more visible to users of the platform, it added.

"As we have done with previous influence operations, we will also preserve these accounts for a time, so that researchers and the public can scrutinise them to see for themselves how these accounts operated," Reddit said.

Mr Johnson was asked about the dossier while visiting a Tory/Lib Dem marginal in Cheadle, near Stockport.

He said: "Well, I do think we need to get to the bottom of that. As far as I'm aware we haven't yet established the truth about that, but what I certainly think is that document, whatever it was intended to prove, did not prove what Jeremy Corbyn and the Labour Party hoped it would prove.

"I'm afraid it was just another distraction from the void at the heart of Labour's policy on Brexit and that still remains with five days to go until this crucial election."

Liberal Democrat leader Jo Swinson expressed concern about claims of Russian involvement in the emergence of the documents.

She said: "All of us should be concerned if a foreign country is trying to interfere in our democracy.

"And that is why it is so appalling that the Prime Minister is sitting on a report that was written weeks before the general election, that the Security Committee say should be published, into interference in UK democracy by foreign countries like Russia.

"He should have published that report.

"He is keeping it secret and it has to lead people to wonder what he has got to hide."

Johnson rejects Corbyn claims over post-Brexit trade

Last night Mr Johnson rejected a claim by Labour leader Mr Corbyn that leaked Treasury documents showed there would be customs checks and restrictions on trade between Britain and Northern Ireland as a result of his Brexit deal.

Mr Johnson and the Labour party leader clashed on Brexit and other issues in their final televised debate of the UK general election campaign.

Mr Johnson insisted that leaving the EU with his Brexit deal next month would allow the UK to improve trade and control immigration. 

"I do find it slightly curious to say the least to be lectured about the union between Great Britain and Northern Ireland by a man who all his political life has campaigned to break up that union and actually supported the IRA for four decades their in their campaign violently to destroy it," he said.

Mr Corbyn said Mr Johnson should show a "degree of honesty" about the implications of the arrangements he had agreed for Northern Ireland.

He said: "He spoke at the DUP conference and said there would be no restrictions whatsoever. We now know there are restrictions. He could and should have said that at the time." 

A snap YouGov poll of 1,322 viewers following last night's the debate narrowly gave victory to Mr Johnson by 52% to 48%, although Mr Corbyn was judged the more trustworthy by a margin of 48% to 38%.

The parties return to the campaign trail today with Labour promising every bus in England will be electric-powered by 2030, reducing emissions by more than 70%.

The Tories are pledging extra cash for grassroots football to bolster the UK and Ireland's bid for 2030 World Cup, with an an extra £550 million over ten years for community sports facilities.

And the Liberal Democrats are promising to bring in "safe standing" areas at top flight football matches, saying it would offer fans more choice, a better atmosphere, and cheaper tickets.