The targeting of Tesco last Saturday, which caused 14 stores to shut, was a reminder to the industry of how vulnerable it is to attack.

With a presence on every high street, 24-hour opening and a constant throng of shoppers, retailers are sitting ducks. Activists don't even need to carry out their threats to cause chaos, as stores can be shut and millions lost with a single phone call.

Last weekend's scare wasn't the first that Tesco has faced. Six years ago father-of-two Robert Dyer was sent to prison for mounting a six-month blackmail campaign in which he demanded the supermarket issue him with special reward cards to be used in cash machines and threatened bomb attacks if his demands were not met.

Other major supermarkets have also been targeted. Between 1994 and 1998, 61-year-old Edgar Pearce, dubbed the Mardi Gras bomber, initially launched an extortion campaign against Barclays Bank, but then switched to blackmailing Sainsbury's. He threatened to kill customers with crossbows if his demands for money were not met.

And it's not just the big stores. Several Asian newsagents have been targeted in the wake of the attack on Glasgow Airport and failed bombings in London.

Manufacturers have also become victims. In 2005, a police inquiry was launched when glass and needles were discovered in Kingsmill bread. Allied Bakeries traced the loaves to its plant in south-east London but is still being intermittently targeted.

The reasons for targeting grocery vary wildly. Pressure groups and activists, usually quite peaceful, target supermarket chains for reasons ranging from perceived exploitation to animal welfare policies, pollution or planning.

In Scotland a few years ago, David Currie, who called himself Lone Wolf, accused Safeway of charging excessive prices for petrol and took matters into his own hands by ordering store managers to cut prices. He sent letters threatening to poison food in-store if his demands were not met.

Then there is the mean-spirited competitor. Five years ago, Michael Hancocks, a major shareholder of Aston Manor Brewery, hatched a plot to pour yeast into ciders produced by Bulmers, so that the product recall would benefit his own company. "The better a company is doing, the more vulnerable it will be," says Professor Cary Cooper, professor of organisational psychology and health at Lancaster University Management School.

It's also remarkably easy to tamper with food. "Someone can easily walk around a supermarket when it's quiet and do what they want to," he says. "But it's even easier to cause disruption by pretending to contaminate a product. If they are being watched by CCTV, the threat seems real."

So what can be done to prevent these attacks? Nigel Pollard, head of communications at Scottish & Newcastle, says that if someone is determined to tamper with products or issue threats, then all you can do is involve the authorities.

Companies can be prepared, however. "Any company worth its salt must have detailed procedures in place so they can be quickly activated should a threat arise," says Pollard. "Those plans should also be regularly put to the test with realistic simulations. That way all relevant staff will be aware of how best to handle the situation."