UK-US trade documents have been leaked last month, showing the ruling Conservative Party's plans to offer the NHS for sale in post-Brexit talks.
The leak of classified UK-US trade documents online, tied to a previous Russian disinformation campaign, has all the hallmarks of an attempt to interfere in Britain's upcoming election, a British minister said on Saturday.
The opposition Labour Party says the leaked documents show the ruling Conservatives are plotting to offer the state-run National Health Service (NHS) for sale in post-Brexit trade talks with Washington.
The NHS, much loved by Britons, has become a major issue in campaigning for the December 12 election, polls ahead of which show Labour trailing the Conservatives.
On Friday, social media site Reddit said it believed the documents had been leaked by a campaign that originated in Russia, fuelling fears that Moscow was seeking to interfere in Britain's election.
"I understand from what was being put on that website that those who seem to know about these things say it seems to have all the hallmarks of some form of interference," culture minister Nicky Morgan told BBC radio.
"If that is the case, that is extremely serious."
Prime Minister Boris Johnson said the truth about the leaks had not yet been established, repeating that the documents did not prove Labour's argument that US President Donald Trump wanted the NHS to be included in a future trade deal.
The government has said it is looking into the matter with support from the National Cyber Security Centre, part of the GCHQ signals intelligence agency.
Researchers told Reuters on Monday that the way the documents were first shared on Reddit and then promoted online closely resembled a disinformation campaign uncovered earlier this year which originated in Russia.
Labour declined to comment on how the party acquired the leaked documents, saying its decision to publish them on November 27 was in the public interest. The documents contain 451 pages detailing six meetings between US and British officials to discuss trade and investment since July 2017.
"We obtained those documents, we believe those documents to be correct and nobody until yesterday had denied the correctness of those documents," Labour leader Jeremy Corbyn told Sky News.
Britain has previously accused Russia of meddling or trying to interfere in Western elections, accusations repeatedly denied by Moscow. Officials in Moscow were not immediately available for comment on the latest incident.
The British parliament's Intelligence and Security Committee (ISC) has carried out an inquiry into allegations of Russian activity aimed at the United Kingdom, including in the 2016 referendum on EU membership, but Johnson declined to publish its report before the election because it needed careful study.
The government says there has been no successful attempt to meddle in British elections but critics have said Johnson sat on the report because it might contain embarrassing revelations about his Conservative Party's links.
"The prime minister has answers to give which he refuses to do about Russian donations to the Tory (Conservative) Party or the report that he is sitting on about Russian interference in British politics," Corbyn said.
"Of course there should be no interference in our British political system by (US President) Donald Trump or the Russians."
Britain's security agencies have warned that Russia and other countries may try to disrupt next week's election – called by Johnson to try to break the Brexit deadlock in parliament – with cyberattacks or divisive political messages on social media.