The package was found to contain ricin and is believed to have come from Canada. It was intercepted at a US government mail facility and did not reach the White House, according to American media reports citing a federal official briefed on the matter.

The White House and US Secret Service have so far declined to comment.

Asked about the reports, the FBI said the agency and “US Secret Service and US Postal Inspection Service partners are investigating a suspicious letter received at a US government mail facility.

“At this time, there is no known threat to public safety.”

Ricin is found naturally in castor beans but it takes a deliberate act to convert the substance into a biological weapon.

If exposed to even the tiniest amount of the poison a person can die within 36 to 72 hours.

There is no known antidote.

In February 2018 the president’s former daughter-in-law was rushed to hospital after opening an envelope containing suspicious white powder.

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In April 2019 a developmentally disabled man from Massachusetts was sentenced to five years of probation for sending white powder to President Trump’s sons, among others.

Daniel Frisiello was spared jail when the judge feared he would not respond well to incarceration.

There have been numerous incidents involving envelopes mailed with ricin to American officials.

In 2018, a Utah man, William Clyde Allen III, was indicted for making ricin-related threats, including mailing a threat against President Trump and other federal officials including FBI Director Christopher Wray.

All the letters were found to contain “castor bean material.”

Mr Allen remains in custody.

Two people were convicted in separate incidents of sending ricin-tainted letters to then-President Barack Obama.

In May 2014, a Mississippi man, James Everett Dutschke, was sentenced to 25 years in prison after pleading guilty to sending letters with the deadly substance to Mr Obama, as well as a US senator and a state judge.

In July 2014, a Texas actor was sentenced to 18 years for mailing letters containing ricin to Mr Obama and former New York Mayor Michael Bloomberg.