Security has been stepped up at Sainsbury's 376 food stores amid fears that a "bullet bomb" could be detonated inside one.
In the past 19 months, the extortionist, who uses the codename "Mardi Gra", has either sent in the post or planted a total of 25 explosive devices - most of them using bullets - against Barclays. Three people have suffered minor injuries in the attacks.
On 10 July, he sent a letter to Sainsbury's head office in south-east London, threatening to leave bombs inside stores or to send them through the post, unless what was described as a "considerable amount" of money was paid. Police have refused to disclose the total demanded, but the bomber is understood to have demanded more than pounds 1m from Barclays.
Previously, it had been thought that Mardi Gra was probably a disgruntled Barclays' customer or a former member of staff, but the shift towards one of the country's best-known supermarkets suggests he is an obsessive criminal who is becoming increasingly desperate. Police now fear he could escalate his campaign.
This concern was further heightened by the disclosure from Sainsbury's that the letter sent to their head office suggested that other chains could be targeted in future. A spokeswoman said: "The letter raises the prospect of including other supermarket companies."
Dino Adriano, Sainsbury's deputy chief executive, added: "There was a demand for money from this extortionist, who also threatened to broaden his previous activities to include supermarkets."
Det Supt John Beadle, of Scotland Yard's Specialist Operations Department, said he believed the likely target area was London and the south east - probably the region where the bomber lives or works. He said that the bullet bombs had been put together with "a considerable degree" of care and time.
The devices were fitted into video boxes and books and include shotgun cartridges and bullets designed to explode when opened. With the exception of the ammunition, most of the components could be bought from DIY shops.
The extortionist's campaign against Barclays began in December 1994, when six devices were sent in the post to branches in London. Two went off, and a woman clerk suffered minor injuries to her hands. Further devices were sent to individual members of Barclays staff, as well as to branches in London. Others were left outside branches. After the campaign became public in April this year, two people were slightly hurt by a device outside Barclays in Ealing, west London.Reuse content