The UK has joined France and Germany in threatening sanctions against Russia for the poisoning of Alexey Navalny.
Dominic Raab said evidence that the opposition leader was poisoned by a military grade nerve agent developed by Russia was now “undeniable”.
The foreign secretary said the UK stood “side by side” with France and Germany, who have also threatened sanctions.
Mr Navalny collapsed on a flight on 20 August but has made a recovery after treatment in Berlin.
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The Organisation for the Prohibition of Chemical Weapons confirmed on Tuesday that nerve agents from the Novichok family were found on samples taken from the Kremlin critic.
Earlier, France and Germany accused Russia of “involvement and responsibility” for the poisoning and announced they would seek to impose EU sanctions on those responsible for what they called “this crime and breach of international norms”.
In a statement, Mr Raab said: “Despite having a clear case to answer, the Russian authorities continue to make no credible attempt to investigate this attack.
“There is no plausible explanation for Mr Navalny’s poisoning other than Russian involvement and responsibility for this appalling attack.”
He added: “We will work together with our international partners to take forward sanctions targeting Russian officials and others who are considered responsible for this crime as well those involved in the development of the Novichok chemical weapons programme.”
Mr Navalny told BBC Russian earlier this week that recovering from nerve agent was a long haul, with sleepless nights and clumsy movements.
But he said he was “doing much, much better” and insisted that eventually he would go back to Russia.