A letter that has tested positive for the lethal toxin ricin or another poisonous substance has been posted to a US Republican senator.

The letter, intended for Mississippi senator Roger Wicker, was intercepted at a centre handling post for the Capitol in Washington DC.

A laboratory in Maryland confirmed the presence of ricin after initial field tests indicated the poison was present.

However, the FBI said additional testing is needed as field and preliminary tests produce inconsistent results, CNN informs.

The letter had a Memphis, Tennessee, postmark and no return address, according to Senate Sergeant-at-Arms Terrance Gainer.

Ricin, extracted from castor beans, is 1,000 times more toxic than cyanide. It can be fatal when inhaled, swallowed or injected, although it is possible to recover from exposure.

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