Russians almost certainly sought to interfere in the 2019 UK general election through illicitly acquired documents, the government has said.
Foreign Secretary Dominic Raab said any attempt to meddle in UK democracy was "completely unacceptable".
The documents - on UK-US trade discussions - emerged online and were used by Labour in the 2019 campaign.
A much-delayed report into allegations of wider Russian interference into UK democracy is due next week.
Labour said it condemned "any attempt by Russia, or any foreign power, to interfere in our country's democratic processes" and pledged to work to protect the nation's security.
This is the first time the government has acknowledged with such certainty that Russians interfered in the UK's democratic processes.
A Downing Street spokesman dismissed as "nonsense" suggestions that the timing of Mr Raab's statement was aimed at pre-empting the publication of the Russia report by Parliament's Intelligence and Security Committee.
At the 2019 election, then-Labour leader Jeremy Corbyn said the documents proved the Conservatives were planning to include the NHS in a future trade agreement with the US - something denied by the government.
Writing on his Facebook page, Mr Corbyn accused the Conservatives of wanting "to distract from the damage a Trump trade deal would do to our NHS by continuing to push the bogus claim Labour received Russian support".
He added that the government's claim "is an attempt to divert attention from the threat to the NHS and the Tory party links to Russian oligarchs expected to be revealed in the long-buried parliamentary Russia report."
The government launched an inquiry into how the papers got into the public domain, with help from the National Cyber Security Centre.
The announcement comes as a group of national security services warn that Russian hackers are targeting organisations trying to develop a coronavirus vaccine.
Despite many suspicions of Russian attempts at meddling in the referendum and other campaigns, significant concrete evidence is in short supply.
So, it matters that this is the first time a UK minister has made an explicit link to Russia, in one way or another, trying to meddle in elections in the UK.
But the timing of that statement creates its own intrigue too.
Next week, at long last, the powerful group of MPs who monitor UK intelligence will publish a report on the Russian threat to the UK - a report that has been anticipated for a very long time and may perhaps set the record straight on all of this.
Is it politically convenient for ministers to acknowledge the threat themselves just before others may make embarrassing claims about it?
Labour politicians have frequently accused the Conservatives of ignoring Russian interference because of their relationship with Tory Party donors.
Did it suit the government to publicise the claims that material used by Labour was also manipulated by Russia?
It seems, as one former UK ambassador to Moscow said, a "remarkable coincidence" that the government decided at this moment to admit explicitly, for the first time, that Russia has tried to stick its nose into our politics - especially when there is a running criminal investigation into who obtained the documents to start with.
But Downing Street denies that there is any link in the timing at all.
In a written ministerial statement, Mr Raab said "the government has concluded that it is almost certain that Russian actors sought to interfere in the 2019 general election through the online amplification of illicitly acquired and leaked government documents.
He said the documents were disseminated online via the social media platform Reddit.
"When these gained no traction, further attempts were made to promote the illicitly acquired material online in the run up to the general election," he said.
The foreign secretary goes on to say that there is "no evidence of a broad spectrum Russian campaign against the general election" but that "any attempt to interfere in our democratic processes is completely unacceptable".
The forum website Reddit said the unredacted papers had been uploaded as "part of a campaign that has been reported as originating from Russia".
It suspended 61 accounts that showed a "pattern of coordination".
Mr Raab's statement is not connected to the Intelligence and Security Committee's report into Russian interference, which is due to be published next week.
The committee launched its inquiry in November 2017 following concern Russia sought to influence the US 2016 election and the 2016 Brexit vote.
After the poisoning of ex-Russian spy Sergei Skripal and his daughter Yulia in March 2018 the investigation became the "primary focus" of the committee.
The committee heard evidence from independent experts as well as the secret intelligence agencies, MI5, MI6 and GCHQ.
BBC security correspondent Gordon Corera said the committee's report has looked into Russian activity from traditional espionage to subversion - with a particular focus on possible interference in the 2016 EU referendum and 2017 general election.
In addition to cyber-espionage and social media campaigns, the report also examines Russian influence through money.
The delay in publication has led to speculation the report contains details embarrassing for the Conservatives - specifically in relation to the party's Russian donors.
However, Commons leader Jacob Rees-Mogg insisted the hold-up was due to a number of committee members leaving Parliament and the need "to make sure that the right people with the right level of experience and responsibility could be appointed".
Russian foreign ministry spokeswoman Maria Zakharova said Mr Raab's statement was "ambiguous" and "confusing".
She said Mr Raab had said there was "no evidence of full-scale interference" by Russia in his statement but had also claimed "any attempts of such interference are unacceptable".