An envelope addressed to Donald Trump has been found to contain a substance suspected to be the highly toxic substance, ricin.

A Secret Service spokesman told CNN “the Secret Service can confirm receipt of a suspicious envelope addressed to the President on 1 Oct, 2018.”

“The envelope was not received at the White House, nor did it ever enter the White House,” the spokesman continued.

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Federal authorities confirmed that two similar packages, also suspected to contain the lethal poison, were mailed to two senior Pentagon officials; defence secretary Jim Mattis and Admiral John Richardson, the chief of US naval operations.

The Pentagon put all of its mail under quarantine yesterday after the packages suspected of containing the poison were identified, The New York Times reports.

Those packages have been turned over to the Federal Bureau of Investigation for further analysis.

“All threats directed towards the President, or any Secret Service protectee, are treated seriously and fully investigated,” the Secret Service said in a statement.

The suspicious packages have caused a heightened level of security on government and military facilities around the United States.

“We are maintaining our vigilance for all packages coming into not just the Pentagon but facilities worldwide,” a defence official told the Washington Post, speaking on the condition of anonymity.

Ricin is a poison “found naturally in castor beans”, Time says, which can be made into a “partially purified material or refined into a terrorist or warfare agent”.

People can become exposed to the poison through the “air, contaminated food or water”, according to the US Centre for Disease Control.