Yvette Cooper says claim against Kremlin ‘deeply serious’ while Russia dismisses western ‘feeblemindedness’

The UK is mulling fresh sanctions against Moscow after pinning blame on the Kremlin for the poisoning of the Russian opposition leader Alexei Navalny, Yvette Cooper has suggested.

The Foreign Office and four of the UK’s allies – Sweden, France, Germany and the Netherlands – announced on Saturday they had determined that Navalny’s death was most likely the result of poisoning using dart frog toxin arranged by the Russian state.

The Russian embassy in London has denied Moscow was involved in Navalny’s death two years ago in a Siberian penal colony and described the announcement as illustrating the “feeblemindedness of western fabulists”.

In a rebuke on the BBC’s Sunday With Laura Kuenssberg programme, Cooper, the foreign secretary, said the accusation against Russia was “deeply serious” and a product of two years of evidence gathering.