A “racist” letter was sent to Prince Harry and his fiancée Meghan Markle, in a package which also contained white powder.
The police said their anti-terror team were investigating after "malicious communications" were delivered to St James’s Palace, in what they have branded a “racist hate crime”.
The substance inside the package was originally treated as possible anthrax, it is understood, but turned out to be non-suspicious.
St James's Palace is home to several members of the Royal Family and their household offices.
The residence is situated around two miles from Kensington Palace, where Prince Harry lives with his soon-to-be wife Meghan Markle.
Ms Markle, whose father is Caucasian and mother is African-American, said in a BBC interview in November that it had been "disheartening" to receive negative attention about her ethnicity in the early period of her relationship with Harry.
A spokesperson for Harry said they would not be commenting on the matter of the package.
A Met spokeswoman said: “Police are investigating after a package containing a substance was delivered to St James's Palace on Monday, 12 February.
“The substance was tested and confirmed as non-suspicious.
“Officers are also investigating an allegation of malicious communications which relates to the same package, and it is being treated as a racist hate crime.
“The matter is being investigated by officers from the Met's Counter Terrorism Command."
There have been no arrests yet but enquiries are continuing.
The letter was sent to the royal residence the day before a package with a similar white powder was sent to Amber Rudd's office.
Prince Harry's team have previously put out an official message to warn the press about “racial undertones” in their coverage of his relationship with Ms Markle after it was claimed that “a line had been crossed”.
The statement issued on behalf of Prince Harry in late 2016 said: “His girlfriend, Meghan Markle has been subject to a wave of abuse and harassment. Some of this has been very public – the smear on the front page of a national newspaper; the racial undertones of comment pieces; and the outright sexism and racism of social media trolls and web article comments," it said.
“Some of it has been hidden from the public - the nightly legal battles to keep defamatory stories out of papers; her mother having to struggle past photographers in order to get to her front door; the attempts of reporters and photographers to gain illegal entry to her home and the calls to police that followed; the substantial bribes offered by papers to her ex-boyfriend; the bombardment of nearly every friend, co-worker, and loved one in her life.”
Additional reporting by Press Association