MODERN civilisation may not be quite as safe as we thought. Britain’s security services have been privately warning their staff that western societies are just 48 hours from anarchy.

MI5’s maxim is that society is “four meals away from anarchy”. In other words, the security agency believes that Britain could be quickly reduced to large-scale disorder, including looting and rioting in the event of a catastrophe that stops the supply of food.

The maxim will provoke debate over whether MI5 is over-egging the threat, partly to justify its rapidly growing budget. It also opens a wider question as to whether civilised societies could so quickly revert to primitive behaviour.

MI5 — whose motto is “regnum defende”, defend the realm — uses the “four meals” rule to assess the threat levels from attacks on strategic installations, such as computer networks and power stations; natural disasters; or widespread strikes and civil disobedience.

There is evidence that the breakdown of order could be caused partly by the first pangs of hunger but more likely by panic.

This could occur if, for example, a bomb attack prevented food getting into an area or if computer systems were attacked by hackers, throwing the electricity, food and water networks into chaos. Alternatively, an attack by biological, nuclear or radiological weapons could result in an area being cordoned off.

It is likely that the people affected would immediately buy up all the food available. As supplies ran out, the public might try to break through cordons or start competing violently for available food.

It is estimated that after as little as four missed meals, a “law of the jungle” would take over, in which citizens resorted to looting or violence to find food.

The possible breakdown in order, MI5 believes, could occur in isolated pockets or, less probably, across the country.

Some experts claim Britain’s food supply network is diverse enough not to collapse quickly in the event of a major disaster; others point to the speed with which the distribution system briefly came to a halt during the fuel price protests in 2000.

Worry over possible disruption to food supplies was one of the issues the government tried to address in its booklet Preparing for Emergencies: What You Need to Know, circulated to all homes in Britain in the summer, which advised households to stock up on tinned food and drinking water to guard against the possibility of a disruption to supply.

Tony Morris, chairman of the London branch of the Emergency Planning Society, which brings together officials involved in preparing for emergencies, said the risk of serious disturbances was low but he did not rule out the possibility of supermarkets being guarded by police or the army.

In a situation where food was in short supply, Morris said it would be a top priority to keep morale up and to restore supplies quickly.

“Food is a priority but how much of an urgent priority it will be in the initial stages depends on what happens. There might be isolated pockets of collapse, but they will be such that they can be quickly restored,” said Morris.

A further challenge facing the authorities, said Morris, would be over whether to give priority to the supply of water, electricity or food. This decision would be made by a government-established strategic command team.

Marinos Elia, professor of clinical nutrition and metabolism at Southampton University, argues that people can quickly snap if they are deprived of food.

“People are socially conditioned to when they eat their meals, so they will begin to get hungry when they are due to have their next meal,” he said.

“The psychological changes that occur in this period are associated with the uncertainly of what might happen and are as important as the physical changes.”

The Home Office would not comment on contingency plans in the event of an attack. However, the government has taken steps to try to ensure supermarket chains preserve the food supply in times of crisis.

A spokesman for the Department for Environment, Food and Rural Affairs said shops had “robust and resilient” business continuity plans to deal with any threat of disruption.

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