A former tax inspector has been jailed for six years for a £1m blackmail plot against supermarket chain Tesco.

Philip McHugh, of Milton Avenue, Clitheroe, Lancashire, sent 76 letters threatening to bomb Tesco stores across Britain last summer.

The 52-year-old also threatened to contaminate Tesco products if they refused to comply with his demand for money. The judge at St Albans Crown Court said McHugh was guilty of a sustained and serious effort to extort money from the chain.

McHugh, who was addicted to online gambling and had debts of £37,000, began his campaign last May with a series of letters to Tesco offices in Dundee.

In the letters McHugh threatened to contaminate food in Tesco stores unless he was paid £100,000.

When this tactic failed, he sent a series of increasingly threatening letters to the supermarket chain's headquarters in Hertfordshire.

He demanded that Tesco transfer £200,000 into his bank account to stop him putting caustic soda in yoghurt sold in the store. When this tactic also failed, McHugh sent hoax bomb warnings to 76 Tesco supermarkets, warning of what McHugh called Black Saturday. He said bombs would go off at the stores on Saturday, July 14.

As a result, 14 Tesco superstores including branches in Ledbury, Herefordshire, Clitheroe, Port Talbot, Grimsby and Glasgow were closed, costing the supermarket chain an estimated £1.4m in lost revenue.

Due to delays in the postal service, the remainder of the letters were not received until after the threat had expired.