Donald Trump Jr’s wife was taken to a New York City hospital as a precaution on Monday after she opened an envelope addressed to her husband that contained an unidentified white powder, police said.

A preliminary test indicated the substance was not dangerous.

Vanessa Trump, 40, opened the letter addressed to the president’s son on Monday morning at their midtown Manhattan apartment. She called 911 and said she was coughing and felt nauseous, police said.

Vanessa Trump said she felt unwellVanessa Trump said she felt unwell (Win McNamee/AP)

The Trump Organisation did not immediately respond to a request for comment.

Detectives from the New York Police Department’s intelligence division and Secret Service agents are investigating.

“The Secret Service and our law enforcement partners in New York City are investigating a suspicious package addressed to one of our protectees received today in New York, New York. This is an active investigation and we cannot comment any further,” Secret Service Special Agent Jeffrey Adams said.

Vanessa Trump, a former model, and Donald Trump Jr have five children, none of whom were home at the time of the incident.

In March 2016, police detectives and FBI agents investigated a threatening letter sent to the Manhattan apartment of Donald Trump Jr’s brother, Eric, that also contained a white powder that turned out to be harmless.

Envelopes containing white powder were also sent to Trump Tower, which served as Mr Trump’s campaign headquarters, twice in 2016.

Hoax attacks using white powder play on fears that date to 2001, when letters containing deadly anthrax were mailed to news organisations and the offices of two US senators. Those letters killed five people.