Eighteen companies across Japan have received envelopes containing what a white powder that authorities have confirmed is highly toxic potassium cyanide along with a note demanding money.

The first letters were delivered to pharmaceutical companies, food manufacturers and the head office of a national newspaper last week, with more arriving over the weekend and in the first two days of this week.

Each of the letters contained a letter purportedly signed by one of the 13 members of the Aum Shinrikyo cult who were executed last year for their roles in the release of sarin nerve gas on the Tokyo subway system in 1995. There is no indication that surviving members of the cult - which is under close police surveillance - are behind the extortion attempt

Each of the letters demanded payment of 35 million Korean Won (£23,877) in Bitcoin, the Mainichi newspaper reported, and contained the threat to create fake drugs or lace food with potassium cyanide and then make it accessible to members of the public.

The letter warned that “a tragedy will happen” if the money was not transferred by February 22.

Police have declined to name the pharmaceutical and food companies that have been targeted, although they are understood to be based in Tokyo, Osaka and Sapporo, in northern Japan.