British Prime Minister Theresa May has said that Russia is to blame for the poisoning of a former Russian double agent and his daughter, rejecting denials from Russian President Vladimir Putin.
It comes after the Kremlin said that Britain must prove Russia's involvement in the poisoning or apologise.
Yesterday, Mr Putin said it was "nonsense" to think that Moscow would have poisoned Sergei Skripal and his daughter Yulia.
Both of them have been in a critical condition in hospital since being found unconscious on a bench outside a shopping centre in the English city of Salisbury earlier this month.
When asked if Mr Putin was lying, Mrs May told Sky News "I am clear that what we have seen shows that there is no other conclusion but the Russia state is culpable for what happened on the streets of Salisbury."
Earlier, Britain's foreign minister said that that Moscow's denials over its involvement in the nerve agent poisoning are growing "increasingly absurd".
"The Russian denials grow increasingly absurd," Boris Johnson said as he arrived for a meeting of EU foreign ministers in Brussels.
"This is a classic Russian strategy of trying to conceal the needle of truth in a haystack of lies and obfuscation.".
Britain has said Mr Skripal and his daughter were poisoned by the Soviet-era 'Novichok' nerve agent and that Russia has been stockpiling it and investigating how such weapons could be used in assassinations.
But Mr Putin has said Russia is falsely accused.
He said that he found out about it from the media and that "Russia does not have such (nerve) agents."
Mr Putin said: "We destroyed all our chemical weapons under the supervision of international organisations, and we did it first, unlike some of our partners who promised to do it, but unfortunately did not keep their promises".
EU foreign ministers meeting in Brussels have strongly condemned the poisoning of Mr Skripal and his daughter, and said they took "extremely seriously" London's assessment that Moscow is to blame for the attack.
In a statement, the ministers said that it offered London its "unqualified solidarity" over the attack.
Welcome EU’s "unqualified solidarity" and support today in Brussels on Skripal case - first use of a military-grade nerve agent in Europe for 70+ years. EU partners clear Russia must provide immediate, full, complete disclosure of its Novichok programme https://t.co/RsGZA7CaAi
— Boris Johnson (@BorisJohnson) March 19, 2018
It said: "The European Union takes extremely seriously the UK government's assessment that it is highly likely that the Russian Federation is responsible.
"The European Union expresses its unqualified solidarity with the UK and its support, including for the UK's efforts to bring those responsible for this crime to justice."
Today, technical experts from the Organisation for the Prohibition of Chemical Weapons are in Britain to collect samples of the nerve agent used in the attack.
Despite the tensions, Mr Putin said Moscow was ready to cooperate with London.
On Saturday, Moscow announced the expulsion of 23 British diplomats in a tit-for-tat response to Britain's decision last week to expel the same number of Russian diplomats from London.
