Jeremy Corbyn has said he would still do business with Vladimir Putin despite "all fingers" pointing towards Russia being responsible for the Salisbury spy incident.
The Labour leader's comments came as the National Security Council was meeting on Tuesday to consider the latest information on the poisoning of Sergei Skripal and his daughter Yulia.
Meanwhile, Russian diplomats expelled from the UK were leaving the country's London embassy.
Mr Corbyn has been forced to defend his stance on the attack after declining to categorically blame the Kremlin for the nerve agent attack on March 4.
His earlier warning not to "rush ahead of the evidence" led to criticism from Conservatives and some Labour backbenchers. However, he reiterated his call for Russia to be sent samples of the substance to discover its source.
He told Radio 4's World At One: "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."
Mr Skripal, a former double agent, and his daughter are still fighting for their lives after being exposed to Novichok in the Wiltshire city.
Mr Corbyn maintained there had to be a relationship with Russia and said he would still "do business" with president Putin if Labour came into power.
"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 told the BBC.
The National Security Council was considering the latest developments in the case and how to respond to Russia's reaction to the expulsion of 23 of its diplomats - who Mrs May said were undeclared intelligence officers.
Moscow announced a tit-for-tat measure of throwing out 23 British diplomats but also said it would close the British Council and shut down the British consulate in St Petersburg.
Mrs May told the Russians to pack their bags last Wednesday and activity around the country's embassy indicated that they were leaving.
People could be seen heading towards a white coach parked inside the gated Kensington Palace Gardens complex in London close to the Russian Embassy.
Hugging each other, holding children, pet carriers, suitcases and bags, at just after 10am a number of individuals left in a procession of vehicles including three cars, five people carriers and three small-sized coaches.
One woman could be seen filming the media which had gathered outside from the front seat of a vehicle, as children could be seen excitedly looking out the window of a coach as it pulled on to the main road.
Foreign Secretary Boris Johnson accused Russia of trying to conceal "the needle of truth in a haystack of lies" over the case - after Mr Putin dismissed the idea of Russian responsibility as "nonsense".
Writing in the Daily Telegraph, the Foreign Secretary said the use of a nerve agent against the Skripals was "very deliberate".
"As Ken Clarke pointed out in Parliament last week, the obvious Russian-ness of the weapon was designed to send a signal to anyone pondering dissent amid the intensifying repression of Mr Putin's Russia," he wrote.
"The message is clear: we will hunt you down, we will find you and we will kill you - and though we will scornfully deny our guilt, the world will know that Russia did it."