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.

She said: “Only the Russian regime had the means, the motive and the opportunity to administer this poison while he was in a Russian prison.”

Analysis of samples from Navalny’s body revealed the presence of a toxin called epibatidine, which is produced by dart frogs in South America. The UK and its allies said Navalny could not have ingested the poison accidentally as it is not found in Russia and is generally produced only by the frogs in the wild.

The five European nations have reported the Kremlin to the Organisation for the Prohibition of Chemical Weapons after accusing Moscow of breaching its conventions.

Cooper said the incident showed that “the cold war peace dividend we all believed in and hoped for has gone, and we need to be ready for Russian aggression continuing towards Europe”.

She suggested further sanctions could follow, telling the BBC: “We continue to look at coordinated action, including increasing sanctions on the Russian regime. As you know, we have been pursuing this as part of our response to the brutal invasion of Ukraine, where we are also coming up to the fourth anniversary of that invasion as well.

“We believe that it is the partnerships that we build abroad that make us stronger at home. It is by acting alongside our European allies, alongside allies across the world, that we do maintain that pressure on the Russian regime.”

Noting that the government would also continue to provide military support to Ukraine and remain vigilant towards other different, hybrid threats, Cooper added: “The other thing that I would say, specifically about Alexei Navalny, is one of the things he said was ‘tell the truth, spread the truth’, because that is the most dangerous weapon of all.”

Elsewhere, the shadow foreign secretary, Priti Patel, warned of an “axis of authoritarianism” comprising Russia, China, North Korea and Iran faced by the UK and other western nations.

Speaking on Sky News’s Sunday Morning With Trevor Phillips programme ahead of making a speech at the Munich Security Conference, the senior Conservative said the US remained a “natural ally” and partner for European powers.

A statement from the Russian embassy in London denied all involvement in Navalny’s death. It said: “There is no reason whatsoever to credit such ‘findings’ by western ‘experts’. As with the Skripal case, there are strident accusations, media hysteria, zero evidence, and a host of questions the accusers would rather ignore. So what was it in the end – poison derived from the skin of a South American frog or novichok?

“We have become accustomed to the feeblemindedness of western fabulists. One must ask what kind of person would believe this nonsense about a frog. Yet what truly shocks is the method now favoured by western politicians – necro-propaganda. This is not a quest for justice but a mockery of the dead. Even after the death of the Russian citizen, London and the European capitals cannot allow him to rest in peace – a fact that speaks volumes about those who instigated this campaign.”