Packages carrying a warning that they contained anthrax disease were received yesterday by a government department and a Scottish university, Scotland Yard said.
One parcel was sent to the Department for Environment, Food and Rural Affairs (Defra) in London. A similar package was received at St Andrews University in Fife, which Prince William is due to attend.
Police said the packages have been linked to a threatening e-mail, which said the parcels did not contain anthrax, a potentially deadly infection, but a harmless substance.
Forensic science analysis by police last night confirmed that the package received by Defra did not contain anthrax. Further tests will, however, be done on the substance.
The e-mail said similar parcels would be sent to a national newspaper and a London hotel, both of which it named. Police said that they had contacted both and neither had yet received such an item.
Scotland Yard said: "The e-mail indicated that four premises across the UK had been sent packages containing a substance. It said the substance would purport to be anthrax but would in fact be harmless." Police refused to name the other two organisations referred to in the e-mail, but The Sun newspaper is believed to be one of them.