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Polonium-210 is being sold along with uranium and cobalt-60 via secret “dark net” websites.

There are fears extremists and terrorists could buy the substances as they seek new ways to launch attacks.

Russian arms dealers are believed to have seized supplies from insecure research labs and nuclear sites.

(Image: GETTY)

Traces of polonium-210 have already been discovered at locations in London visited by KGB spy Litvinenko.

It was also found on two British Airways flights used by his alleged assassins.

A microgram of Po-210 the size of a speck of dust could deliver a fatal dose.

Huge amounts were found in the body of Litvinenko after he drank tea laced with the chemical in November 2006.

Experts warn it is one of the most toxic substances on earth and a “trillion times more deadly than cyanide”.

Poisoning victims rapidly lose their hair before their vital organs start to shut down.

Yet our investigators got access to several secret websites listing it for sale.

On one hidden invitation-only site, pictured below, which we are not naming, a vendor advertises various lethal chemicals.

They include Polonium-210 costing the equivalent of £420, with the catchline: “Fresh Po-210. If you don’t know what it is you don’t need.

"Can get large quantity if needed. Send private message before making a purchase.”

Our undercover investigator asked the seller, who said he was from Russia but would not specify where, if he could ship to the UK.

He replied: “Yes.”

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“It is safely packaged and we ship to special pick-up points where a driver will collect.

"Cannot say where for obvious reasons. Please provide postal address and specify required amount.”

Asked where the radioactive chemicals come from, he said: “Reactor sites in the former Soviet Union. Much left over.”

He was also selling radioactive cobalt-60 and uranium, a Geiger counter nuclear detector, an anti-radiation suit and a nuclear material storage container.

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In his feedback section one buyer wrote: “Item was shipped exactly as described and quick.”

Last night a British security source told us: “We already know with the assassination of Alexander Litvinenko and subsequent findings across London that nuclear material and radioactive chemicals are in circulation.

“The dark net is the perfect market for dealers to get rid of it and make money in the process. There are only a select few who want to buy this stuff – killers and terrorists. This illicit and deadly trade just plays into their hands.”