Police say they are treating a "malicious communication", thought to be addressed to Prince Harry and his fiancée Meghan Markle, as a racist hate crime.
Scotland Yard confirmed they launched an investigation after a package was delivered to one of the royal households on Monday, February 12.
The package contained a "substance", which was "tested and confirmed as non-suspicious".
The package was delivered the day before Prince Harry and Meghan Markle travelled to Scotland to meet crowds of people at Edinburgh Castle.
It was intercepted at St James Palace but is believed to have been addressed to Harry and Meghan - who are getting married in May.
On the day the item arrived at the Palace, the royal couple had just released more details of their wedding day - including a two-mile carriage procession through the streets of Windsor.
Officers also say they are "investigating an allegation of malicious communications which relates to the same package, and it is being treated as a racist hate crime".
Packages containing substances do spark fears of anthrax and trigger major security alerts.
However, neither Kensington Palace, where Harry and Meghan live, nor the Metropolitan Police has confirmed if this was the case.