Jeremy Corbyn has been mocked by own MPs after saying Russia should be given a sample of the nerve agent used in the Salisbury attack so it can "say categorically one way or the other" whether it is responsible.
The Labour leader also said he would be happy to work with President Putin if he was Prime Minister and stopped short of blaming the Kremlin for the attack, despite his deputy John McDonnell doing so over the weekend.
It exposes a deepening split in the party's position on the nerve agent attack which has left Sergei Skripal and his daughter Yulia in a coma in hospital.
Last week Mr Corbyn was criticised for refusing to categorically blame Russia for the Novichok poisoning and his communications chief drew further ire when he claimed British intelligence cannot be trusted after the Iraq war dossier.
Responding to his latest remarks on the issue Mr Corbyn was roundly mocked by his own MPs including Ian Austin, a member of the Foreign Affairs select committee, who joked that Mr Putin would never accuse himself of having smuggled the nerve agent into the UK to use against the former spy.
He said: "Does anyone seriously think Putin will say: ‘Thanks for the sample. We have now examined it and yes, I'm sorry to say that it did come from Russia and was then given to one of our agents to murder Mr Skripal in the way we have murdered lots of other opponents’? Who thinks that?”
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Speaking to the BBC's World and One programme Mr Corbyn said: "All fingers point towards Russia's involvement in this, and obviously the manufacture of the material was undertaken by the Russian state originally.
"What I'm saying is the weapons were made from Russia, clearly.
"I think Russia has to be held responsible for it but there has to be an absolutely definitive answer to the question where did the nerve agent come from? I asked the Russians be given a sample so that they can say categorically one way or the other."
In a move that is likely to spark further frustration among Labout MPs Mr Corbyn maintained there had to be a relationship with Russia and said he would still "do business" with president Putin if Labour came to power.
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"Would I do business with Putin, sure? And I'd challenge him on human rights in Russia, challenge him on these issues and challenge him on that whole basis of that relationship," he said.
John Woodcock, chair of Labour's backbench foreign affairs committee, warned allowing Russia to test the poison would be "like saying you trust the fairness of Putin's re-election because he told you it was fine".
He added: "Russia denies every single assassination attempt on foreign soil, no matter how blatant.
"In what parallel universe would we think sending Putin's regime a sample of their poison would lend more credibility to this latest denial?"
It came as Theresa May chaired a meeting of the National Security Council to discuss additional sanctions on Russia after the Kremlin ejected 23 British diplomats from the country. 23 Russian officials, thought to be undeclared spies, left the UK today.
The Prime Minister told the meeting: "There are other measures that government and security officials are actively considering and stand ready to deploy at any time."
She revealed action has been taken at the UK border to beef up visa checks, particularly for private flights, and amendments to the sanctions and money laundering bill are also taking shape.
A spokesman for Mrs May said: "The Prime Minister reiterated that we will freeze Russian state assets wherever we have evidence that they may threaten UK persons or property. And, led by the National Crime Agency, we will continue to bring all the capabilities of the UK law enforcement to bear against serial criminals and corrupt elites."