Foreign Secretary Boris Johnson has described Jeremy Corbyn as the "Kremlin's useful idiot" over his response to the Salisbury poisoning.
He said the Labour leader was giving Moscow "propaganda" false credibility by refusing to "unequivocally" back the government's view it was responsible.
Labour said Mr Johnson had "made a fool of himself" by misrepresenting what he was told by chemical weapons experts.
It stressed Mr Corbyn had repeatedly said the evidence pointed to Russia.
'Avalanche of lies'
Moscow has denied the UK government's claim that it was behind the attack on former Russian spy Sergei Skripal, 66, and his daughter Yulia, 33, on 4 March.
Writing in the Sunday Times, Mr Johnson said the Kremlin had made a "cynical attempt to bury awkward facts beneath an avalanche of lies and disinformation".
He said the Russian government and state-owned media had invented 29 theories about the attack.
"There is only one thing that gives the Kremlin succour and lends false credibility to its propaganda onslaught. That is when politicians from the targeted countries join in," Mr Johnson said.
"Sadly, I am driven to the conclusion that Jeremy Corbyn has joined this effort."
A Labour Party spokesperson said: "Jeremy Corbyn has repeatedly said the evidence points to Russia being responsible, directly or indirectly, and that the Russian authorities must be held to account on the basis of evidence."
He added Mr Johnson had "made a fool of himself and undermined the government by seriously misrepresenting what he was told by Porton Down chemical weapons experts.
"These ridiculous insults won't distract attention from the fact that he has clearly misled the public over vital issues of national security."
Last week Mr Johnson and Mr Corbyn also clashed over the Salisbury case.
Mr Corbyn implied the foreign secretary had exaggerated the findings of the UK's defence laboratory Porton Down on the source of the Novichok nerve agent used in the attack.
Mr Johnson, who during the 2017 general election campaign referred to Mr Corbyn as a "mutton-headed, old mugwump", said the Labour leader's remarks were "lamentable".
A diplomatic crisis between Russia and the West has followed the attack in Salisbury, with more than 20 countries expelling Russian envoys in solidarity with the UK.
On Saturday, the Foreign Office described a Russian request for a meeting with Mr Johnson on the Salisbury poisoning as a "diversionary tactic".
The Foreign Office said it would respond to the invitation "in due course".
It followed Russian Embassy criticism of the UK government's refusal - on immigration rules - to grant a visa to Ms Skripal's cousin, Viktoria Skripal, to visit Britain.
The embassy says the refusal to allow Viktoria Skripal to visit her cousin and uncle was "disappointing" and "politically motivated".
Mr Skripal was jailed by Russia for spying for Britain, but released as part of a spy swap between the US and Russia in 2010.
Yulia was visiting him in the UK when the attack happened on 4 March. She is now conscious and talking in hospital.
Salisbury District Hospital has said Mr Skripal, is responding well to treatment and "improving rapidly".