Media playback is unsupported on your device Media caption The letter to Senator Roger Wicker was detected during a routine inspection of mail

A letter that has tested positive for the lethal toxin ricin or another poisonous substance has been posted to a US senator, Senate Majority Leader Harry Reid has said.

The letter was intended for Roger Wicker, a Republican senator representing Mississippi, Mr Reid said.

It was intercepted at a centre handling post for the Capitol in Washington DC, US media report.

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.

'Memphis postmark'

Mr Wicker issued a statement on Tuesday acknowledging the letter.

"This matter is part of an ongoing investigation by the United States Capitol Police and FBI," he said.

"I want to thank our law enforcement officials for their hard work and diligence in keeping those of us who work in the Capitol complex safe."

An official quoted by AP news agency said two tests had shown positive for ricin but the results are not deemed conclusive without further testing.

Can be fatal when inhaled, ingested or - most dangerously - injected

One to three castor beans chewed by a child, or just eight seeds chewed by an adult, can be fatal

The toxin is part of the waste produced when castor oil is made Q&A: What is ricin?

The letter was detected during a routine inspection of mail and did not reach the US Capitol or Senator Wicker's office, a Senate leadership aide was quoted as saying.

Senators were informed of the letter at a closed-door briefing by FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano about the Boston Marathon bombings, the aide added.

The BBC's Jane Little in Washington says there is a heightened sense of alert in the capital after the attacks in Boston but there is no indication so far of any connection between the two incidents.

It is also not clear why the letter was sent to Senator Wicker.

The Senate's chief security office told Reuters: "The exterior marking on the envelope in this case was not outwardly suspicious, but it was postmarked from Memphis, Tennessee."

All mail sent to members of Congress has been screened off-site since letters laced with anthrax were sent to Capitol Hill in 2001.

In 2004, three Senate office buildings were shut after tests found ricin in letters that had been sent to the Senate majority leader's office.

Ricin was the poison used for the infamous murder of Bulgarian dissident Georgi Markov in September 1978.

He was waiting at a bus stop near Waterloo Bridge, London, when a stranger jabbed him in the leg with an umbrella.

The umbrella injected a tiny ricin-filled pellet into Mr Markov's leg and he died three days later in hospital.