Fifty-one year-old Robert Dyer was jailed for 16 years yesterday, as he pleaded guilty to an "evil" six-month campaign against UK grocery major Tesco at Dorchester crown court. A father of two, Dyer admitted nine counts of blackmail and one count of assault by letter bomb, and said that he had been inspired to commit the crimes by an article in Reader's Digest magazine.

From last August, Dyer modelled a blackmail campaign on Rodney Witchelo's 1998 activities against grocery chains and food manufacturers (detailed in the Digest's "How to catch a blackmailer" article). While serving as a police officer, Witchelo planted razor blades in baby food and mercury in a tin of beans.

Dyer, from Bournemouth, Dorset, sent numerous letters under the pen name "Sally" to a Tesco store in nearby Ferndown, threatening to escalate letter bomb attacks on Tesco consumers if he was not given a reward card that would allow him to withdraw £1000 at a time from cash machines.

"Follow my instructions or your customers will be at risk. It is not my wish to hurt anyone but I will do whatever is necessary to get what I want," read the first letter, which was read out in the courtroom.

Despite the fact that the store created 100,000 special reward cards in response to the blackmail threats, and placed them in every copy of a local newspaper, Dyer sent letter bombs to four Tesco customers. The first three never received the packages, which were retained at the local royal mail sorting office because Dyer had failed to put the correct postage on them. The last exploded in the face of a woman in her seventies.

Dyer, who mis-spelt "without predujice" at the top of every letter he sent, was caught after he left a copy of his first letter in a newsagent's shop. Police believe that he then attempted to set light to a post-box in a bid to destroy the letter.

Prosecuting, Derwin Hope commented: "The evidence shows clearly that this is a very devious man. What it revealed is a deliberate, sustained and indeed cunning blackmail that was only stopped by massive police resources."