Russian opposition leader Alexei Navalny was killed using a deadly toxin found in poison dart frogs in South America, the UK and some of its European allies have said.

Traces of epibatidine were found in samples from Navalny's body and are highly likely resulted in his death in a Siberian penal colony two years ago, the UK Foreign Office said.

The allies said only the Russian state had the "means, motive and opportunity" to deploy this lethal toxin.

The Kremlin dismissed the finding as "an information campaign", according to Tass news agency.

What is the toxin?

Epibatidine is a natural neurotoxin isolated from the skin of the Ecuadorian poison dart frog, according to toxicology expert Jill Johnson.

It was "200 times more potent" than morphine, she told BBC Russian.

Epibatidine can be found naturally in dart frogs in the wild in South America as well as being manufactured in a laboratory.

Dart frogs in captivity do not produce this toxin and it is not found naturally in Russia, the European allies said in their statement.

Species known as Anthony's poison arrow frog and the Phantasmal poison frog are among those that secrete the toxin onto their skin.

Although epibatidine has been investigated as a painkiller and for relief from painful inflammatory lung conditions, it is not used clinically because of its toxicity.

How does dart frog poison work?

This powerful chemical compound acts on nicotinic receptors in the nervous system, according to Johnson.

Because it overstimulates these nerve receptors, if dosed correctly, it can cause muscle twitching, paralysis, seizures, slow heart rate, respiratory failure and ultimately death, she explained.

Alastair Hay, professor of environmental toxicology at the University of Leeds, told PA that its effects can result in breathing being blocked, and that "any person poisoned dies from suffocation".

The toxin being found in someone's blood "suggests deliberate administration", he added.

Epibatidine toxicity can even be "increased by co-administration of certain other drugs and these combinations have been researched", Hay said.

How rare is the toxin?

Epibatidine is extremely rare and found in only one geographic region and only in trace amounts, Johnson said.

It is understood the dart frog referred to by the UK Foreign Office and others was Anthony's poison arrow frog, a species endemic to Ecuador and Peru.

The frogs produce the chemical through eating the right foods to produce alkaloids - a type of organic compound - that make epibatidine and accumulate it in their skin. If the frog's diet changes, its epibatidine reserves will be depleted.

"Finding a wild frog in the right place, eating exactly the food needed to produce the right alkaloids, is almost impossible... almost," Johnson said.

"This is an incredibly rare method of human poisoning. The only other cases of epibatidine poisoning I know of were laboratory-based and non-fatal."

What has Russia said?

European laboratories confirmed Navalny died from the obscure poison, the allies said on Saturday.

Moscow has previously claimed Navalny died of natural causes, though Navalny's widow Yulia Navalnaya has consistently argued her husband was "murdered" by poisoning.

The Russian embassy in London denied Moscow was involved in Navalny's death and described the announcement as "feeble-mindedness of Western fabulists" and "necro-propaganda".

Kremlin spokesperson Maria Zakharova was quoted by Russian state-run news agency Tass as saying: "All the talks and statements are an information campaign aimed at distracting attention from the West's pressing problems."

At the time of his death, Navalny had been in jail for three years and had latterly been transferred to an Arctic penal colony.

According to Russian accounts, the 47-year-old took a short walk, said he felt unwell, then collapsed and never regained consciousness.