Andy Burnham believes the 2016 Brexit referendum result was "legitimate", Downing Street has said, after the prime minister claimed there was Russian interference in the vote.
It is "inconceivable" that Russia did not try to use the referendum to its advantage, a No 10 spokesman said, but the PM believed it did not succeed in influencing the result.
Burnham told The Rest is Politics podcast,, external that upon arriving in Downing Street, he had asked if there had been Russian interference in the vote, adding: "I've had the clear answer that, yes, there was."
Reform UK leader Nigel Farage called the prime minister a "liar" while the Conservatives said he had been "at best... carelessly vague".
Liberal Democrat leader Sir Ed Davey backed Burnham, posting on social media: "This statement needed to be said by a prime minister. After the Conservatives suppressed the truth for years, this took courage."
In his podcast interview, Burnham said there was "certainly misinformation, disinformation that is emanating from Russia into German politics".
He added: "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."
The prime minister did not go into further detail about the nature or extent of Russian activities during the 2016 referendum.
Speaking to broadcasters on Monday morning, Foreign Minister Stephen Doughty would not provide specific details about interference in the vote but said Russia had been "attempting to interfere in a series of democratic events".
Later on Monday, after being pressed several times on whether the PM believes the Brexit result was legitimate, the No 10 spokesman said: "Yes we believe the referendum result was legitimate."
He added: "The PM absolutely accepts the referendum result."
The prime minister's initial comments on Russia prompted an angry reaction from some Brexit supporters.
Farage, who was a prominent Brexit campaigner, said: "The prime minister is a liar, yes, a liar."
"There was no Russian interference - the Russian hoax has run all the way through this," he added.
"This is out-of-touch, out-of-date Andy Burnham with all sorts of bizarre ideas."
Conservative shadow minister Katie Lam said the prime minister "risks sowing doubt about the result of a professionally-run, independently-monitored democratic vote".
In a letter to Communities Secretary Angela Rayner, Lam also suggested Burnham could purposely be seeking to cast doubt on the legitimacy in order to justify his efforts to build a closer relationship with the EU.
In 2020, a parliamentary report did not reach a firm conclusion on whether Russia had tried to intervene in UK elections and referendums, but criticised the government for failing to fully assess and counter the threat from Russian 'troll' accounts on social media.
Report chair Stewart Hosie accused the government of having "actively avoided" looking for evidence of interference.
Asked in 2017, if he had seen evidence of Russia successfully meddling in UK elections, then-Foreign Secretary Boris Johnson said: "Nyet [Russian for no], not a sausage."
Earlier this year, the government published an independent review, external on foreign influence in UK politics which said there were attempts to "sow distrust and exacerbate divisions in UK society with the ultimate aim of undermining confidence in our democracy".
Report author Philip Rycroft said it was difficult to establish the impact of these efforts and that the public "can be credited with sufficient common sense to see of the more egregious attempts to suborn their opinions".
However, he added that while there was no "immediate crisis" in British democracy "there is no guarantee that this will persist".