Andy Burnham has said Russia did interfere with the Brexit referendum and warned that similar activities were taking place in France and Germany.
The prime minister told The Rest is Politics podcast, external that upon arriving in No 10, he had asked about possible Russian interference in the 2016 vote, adding: "I've had the clear answer that, yes, there was."
"I think we've been too much on the back foot on that actually and we haven't informed people sufficiently about the extent to which people are seeking to disrupt the political process from overseas," he added.
Burnham did not provide further details about the type or extent of Moscow's activity during the Brexit referendum.
Britain Unbound, a pro-Brexit campaign group, said Burnham's comments were "another crude attempt to undermine the legitimacy of the decision that the British people made in 2016".
It said that foreign interference in UK elections was "obviously unacceptable" but added: "There is no good reason to believe that Russian intervention influenced the outcome of the referendum in 2016.
"It is also insulting to suggest that voters were so easily duped."
In his interview, due to be released on Monday, Burnham said he had discussed the problem of Russian interference with Chancellor Friedrich Merz during his visit to Germany earlier this week.
"There is certainly misinformation, disinformation that is emanating from Russia into German politics," he said.
He continued: "These things are important now to put clearly on the record and I believe help voters in Germany and France understand that some of this politics is instigated via our enemies, namely Russia."
It is the first time a UK prime minister has said Russia sought to sway the 2016 Brexit poll but there have long been warnings about efforts by the Kremlin to exert influence on UK politics.
In 2020, Parliament's Intelligence and Security Committee published a report into Russian interventions in UK elections and referendums.
It did not provide a definitive conclusion on whether Russia had attempted to influence the results of the EU referendum but did criticise successive governments for failing to fully assess or counter the threat.
In response, the Russian foreign minister dismissed the report out of hand and called it "Russophobia".
And a Kremlin spokesman said Russia did not meddle in other countries' elections.
In the same year, however, the then-Conservative government said it was "almost certain that Russian actors sought to interfere" in the 2019 general election.
BBC Verify has tracked Russian-linked disinformation campaigns including using AI to create fake videos which impersonate journalists and news organisations reporting on Ukraine.
One fake news website falsely claimed that Ukrainian President Volodymyr Zelensky had bought a mansion owned by King Charles at a discounted price.
Earlier this year, Vijay Rangarajan, head of the UK Electoral Commission, told MPs that Russian deepfake videos had been "used extensively in elections around the world".
"There is no reason to assume Britain would be an exception," he added.
Last month, during his speech to the United Nations, Burnham accused Russian agencies of using "every means at their disposal to spread lies and disinformation and prey on people's fears".
He announced that the UK would set up a new centre to "stem the poisonous tide" of disinformation.
A Downing Street spokesperson said: "We are focused on ensuring our citizens and our institutions are protected from foreign powers and their proxies seeking to threaten lives and undermine our democratic values.
"We are doing this by strengthening our powers through the National Security Act, strengthening the resilience of our institutions and critical national infrastructure, strengthening our borders, and strengthening our police and intelligence agencies."