Cashpoints freezing, savers besieging banks, riots, shortages - a euro zone breakup could bring chaos. But while some governments are tentatively thinking about such an eventuality, albeit a remote one, others are holding back.

Preparations at government and official level appear much less advanced than in the business sector, where risk managers have begun to think what was once unthinkable.

Those with knowledge of government thinking say the main emphasis so far is on the banking sector - identifying, protecting and supporting the institutions most exposed. Not everyone is convinced that is enough. Debt and currency crises in Argentina and elsewhere, they say, suggest the shock would extend well beyond the financial sector.

This is not just a high finance issue, said one British official specialising in security and resilience planning. Real lives could be pitched into traumatic changes. The next flash of financial crisis may hit real political economies within hours or days.

Even if only one country - say Greece - were to leave the euro zone, the panic over other heavily indebted states such as Italy and Spain could lead to bank runs and a cash exodus across the border. Financing of ordinary commercial activities might dry up. Airlines might be grounded.

The speed with which worries over liquidity turned into a run on British bank Northern Rock in 2007 give an idea how fast things can escalate. Queues formed outside branches and only nationalisation and a government guarantee of deposits restored calm.

Logistics and law enforcement also need serious thought, experts say. Capital and border controls might need to be tightened as states leaving the euro try to replace one currency with another. While the public might tolerate capital controls, ATMs shutting will not be tolerated, said the British official, speaking on condition of anonymity. The key to managing crises is communication. Central bankers are not good at communicating with the masses. It is time political leaders began preparing their own communication strategies.

If such preparation is happening, countries are keeping it extremely quiet. For now, the central strategy of France, Germany and others looks to be to carry on, hope this week's European summit marks a turning point and avoid thinking of the alternative as much as possible. In Germany, most officials simply refused to comment on any contingency plans. No one, they said, seriously expected the currency to break up and so there was little point in considering it.

This does not reflect our thinking or our expectations. We are concentrating all of our efforts on resolving the problem, said a German government official, speaking on condition of anonymity.

PURE SCIENCE FICTION

Within the euro area, official discussion of the topic was completely taboo until French President Nicolas Sarkozy and German Chancellor Angela Merkel raised the possibility of a Greek euro zone exit last month. The survival of the currency came first, they said.

That immediately released the shackles on governments to contingency plan for that reality, says Alastair Newton, a former UK Cabinet Office and Foreign Office official and now chief political analyst for Japanese bank Nomura. Before that, they couldn't plan because if they did it would leak out and then it could become a self-fulfilling prophecy.

Most official sources and spokespeople in the military declined comment. The government contemplates every situation imaginable and has all kinds of contingency plans for a wide range of possible threats, but we don't have one specifically for any social unrest as a result of the euro zone crisis, said a source from the Spanish interior ministry. I can't make up science fiction.

A French police union spokesman said he had no doubt that some form of planning had been done. He said it would be impossible to up police presence around banks and tackle unrest without pulling resources from other duties such as counterterrorism. A spokesman for the French police service itself did not reply to a request for information.

Greek police spokesman Thanassis Kokkalakis said no specific plan had been pulled together linked to a currency exit, but that with officers facing demonstrations, unrest and violence every day over austerity measures, they were more than prepared.

Theoretically, if such an event occurs, we are ready, he said. We don't need to do anything more. We have a standard action plan because we have to deal with this issue very often.

For Nomura's Newton, that approach is too complacent. But he also believes euro sceptics in euro zone outsider Britain may be getting carried away. I think what you're seeing in Britain has much more to do with euro sceptic feeling than the genuine situation. If you go to the euro zone, no one honestly thinks it is going to break up and so it's hardly surprising there is less planning.

KEEP CALM, MUDDLE THROUGH

British newspapers have been full of stories about Foreign Office planners drawing up emergency schemes to assist or even evacuate Britons from a collapsing euro zone. Government officials declined comment on specific stories but some said they were re-examining long-held assumptions about Europe's stability.

Britain's chief of defence staff General David Richards said earlier this month he was pondering the potential security implications of the euro zone crisis. Last week, British National Security Adviser Peter Ricketts addressed the same topic. It is something we will be discussing - the potential national security implications - in the next week or so.

I think what we're looking at would be a lot of muddling through type solutions, says David Lea, Western Europe analyst for London-based risk consultancy Control Risks. There's a limit to how much you can plan for this kind of situation, even if the work has actually been done.

(Additional reporting by Mohammed Abbas and William Maclean in London, Angeliki Koutantou and Renee Maltezou in Athens, David Brunnstrom in Brussels, Tracy Rucinski in Madrid, Nicholas Vincour in Paris and Stephen Brown in Berlin; editing by Janet McBride)