The number of threatening letters containing white powder which have been delivered to addresses all over the Netherlands has reached 19, police said on Friday, as they published a photograph of one of the envelopes.

The blue envelope includes a neatly written return address – that of insurance company Nationale Nederlanden in The Hague.

Most of the letters – 12 – were sent to addresses in Amsterdam, with others delivered in Amersfoort, Arnhem, Best, Rotterdam, Utrecht, Roermond and Zeist. The recipients include hotels, hospitals, law firms and publishing companies.

The letters are now being examined by the national forensic institute who are also trying to identify the powder. No-one has been hurt so far, but the powder has caused slight breathing problems and burning eyes in some recipients, police said.