A terror threat is being investigated at Kensington Palace after a letter containing white powder was sent to Meghan Markle and Prince Harry.

The letter has prompted a security scare despite it being intercepted before reaching the couple, with it being feared that it could contain anthrax, according to The Evening Standard.

Markle, 36, and Harry, 33, are said to have been informed of the incident, and tests have since revealed that the powder was harmless. Scotland Yard is now said to be on high alert for the royal wedding in May.

Terror police are reported to have been called to the palace after the letter was sent on 12 February – just one day before they revealed a suspect package had been sent to the couple.

Markle and Harry's spring wedding, which will take place on 19 May in St George's Chapel at Windsor Castle, is believed to set back the royal household over £10m for security.

IBTimes UK previously revealed the predicted cost of the royal wedding. Founder of wedding planning website and app Bridebook, Hamish Shepard, claimed that the ceremony would cost the Royal family a cool £1.5m, without security.

He said: "Meghan and Harry's wedding is expected to cost £1,463,273 [$1,964,312], and that's not including the honeymoon or the £10m expected security costs involved on top. Their wedding will cost almost 100 times the average cost of a wedding. Their highest expenditures are likely to be the catering, drinks and the decor."

meghan markle
Prince Harry and fiance Meghan Markle leave the 'Endeavour Fund Awards' Ceremony at Goldsmiths Hall on February 1, 2018 in London, England. The awards celebrate the achievements of wounded, injured and sick servicemen and women who have taken part in remarkable sporting and adventure challenges over the last year Getty