Scotland Yard is investigating after a letter containing white powder purporting to be anthrax was sent to Meghan Markle at Kensington Palace.

The letter, addressed to her and fiancé Prince Harry and understood to contain a racist message, led to a security scare, with specialists rushed in to check the powder.

Measures already in place meant the package was intercepted before it reached the couple. Meghan, 36, and Harry, 33, are understood to have been informed.

Officers from the Metropolitan Police’s counter-terrorism command were called in after the letter was received on February 12. It was delivered to St James’s Palace for sorting. Analysis of the powder found it was harmless.

Thames Valley Police has bought a net of spikes which can stop a vehicle weighing up to 17 tons and be deployed in less than a minute.

Meghan and Harry are due to ride around a public route in a carriage after marrying at St George’s Chapel in Windsor Castle on Saturday May 19. They will pass along a route including Castle Hill, High Street, Sheet Street, Kings Road, Albert Road and Long Walk then return to the castle.

There are no plans to change any of the details of the wedding, which is being paid for by the royal family.

The anthrax scare is the first security alert surrounding Harry’s fiancée, who will become an official member of the royal family after the wedding but already has 24-hour protection. She has been assigned a team from the Met’s royalty and specialist protection command.