Britain had stores of the Novichok nerve agent before the poisoning of former spy Sergei Skripal, the Russian ambassador to London has claimed.
In an extraordinary and lengthy press conference, Alexander Yakovenko said the accounts around the events in Salisbury were so complicated it would take someone like fictional detective Hercule Poirot to solve the crime.
Mr Skripal and his daughter Yulia are in a critical condition after collapsing on a bench in Salisbury on 4 March. Britain accuses Russia of attempted murder but Russia denies involvement and says Britain has no evidence.
"We don't have any information," Mr Yakovenko told reporters. "The investigation is classified. We don't know the motivation of the British government, and that's why it says: this case is so complicated, we need, let's say, some wisdom of a person like Poirot to investigate."
The ambassador said Russia "can't take British words for granted" and that the UK had a "bad record of violating international law and misleading the international community".
"History shows that British statements must be verified. We demand full transparency of the investigation and full cooperation with Russia," he added.
Mr Yakovenko even cast doubt on British assertions Novichok was the nerve agent used in the poisoning, claiming Russian specialists thought the pair would already be dead if so.
Despite Moscow's claims of innocence, Lithuania's president offered her full support to Theresa May and the British government over its stance on the poisoning attack.
At a European Union summit in Brussels, Dalia Grybauskaite said she was "considering" expelling Russian diplomats from the country - a former Soviet state which shares a border with Russia's western Kaliningrad enclave.
Ms May told EU leaders they must unite to counter the threat from Russia, and said the Salisbury incident "was part of a pattern of Russian aggression against Europe and its near neighbours from the western Balkans to the Near East".
Addressing the fallout over Cambridge Analytica, Mr Yakovenko insisted Russia had no links to the company at the centre of the Facebook data scandal.
Asked why he was pictured alongside Alexander Nix, the suspended chief executive of the firm, he said: "One day I was invited to the Windsor (polo) club, and the main prize was the Russia vodka, Ivan the Terrible. And the organisers asked me... why don't you give the prize to all the members of the team."
"I gave the prize to maybe 10 people, 12 people, and that was the only time that I met this gentlemen. But the picture is good, I like it."
Meanwhile, the British Council said in a statement it had cancelled all events and programmes in Russia after being told by its foreign ministry to cease activity.
"We deeply regret this and are grateful for your understanding," the statement said.
The British Council, a state-funded body which promotes British culture overseas, has worked in Moscow continuously since 1959.
Additional reporting by agencies
