Thousands of Russian Twitter accounts posted pro-Labour messages in the run up to last year's general election, it has been claimed.
Many of the 6,500 accounts are said to have been 'bots' that tweeted using female English names, but were in fact mass-produced accounts pumping out messages in support of Labour and Jeremy Corbyn.
The claims are made by the Sunday Times, which carried out research in conjunction with Swansea University.
Nine out of 10 messages from the Russian accounts backed Labour's campaign, according to the report, which also claims 80% of the automated accounts had been created in the weeks before the June vote.
Culture and digital secretary Secretary Matt Hanock called the reports "extremely concerning".
"It is absolutely unacceptable for any nation to attempt to interfere in the democratic elections of another country," he said.
Labour has said it was "not aware of any from automated bots, categorically did not pay for any and are not aware of any of our supporters doing so".
In a statement, it said its "people-powered election campaign" was the reason for its success and that many of its policies run contrary to "both May and Putins conservative philosophy and their super-rich supporters".
It added that the Russian government had actually backed the Tories during last year's campaign, with the London embassy promoting "Theresa May's 'strong and stable' slogan".
Shadow Chancellor John McDonnell also told Sky News the allegations were "ludicrous", "farcical" and a "classic Sunday Times smear campaign".
The Sunday Times report claims that the Twitter bot accounts fired up at critical points during the election battle.
For example, criticising The Tories manifesto on its launch day, retweeting support for Mr Corbyn's rallies, criticising the PM over police numbers after the Manchester bombing, and urging Labour supporters to vote on polling day.
Mr Corbyn was recently criticised by some for his caution in blaming Russia for the Salisbury nerve agent attack.
In all, the Times said its researchers found that 16,000 Russian bot accounts had tweeted about UK politics since April 2017.
For the election analysis, it honed in on 20,000 tweets from Russian in the four weeks before the general election and looked at their "political slant".
The claims mirror the ongoing investigation into allegations that Russia also mobilised social media to whip up support for Donald Trump during the 2016 US election.
Russia has repeatedly denied those claims.
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Mr Hancock said the Government was "already assessing the evidence of online interference in our democracy".
"We will not stand by when attempts are made to try to undermine our democratic way of life. Social media companies should reveal what they know," he added.
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