IT'S been a week since shares in Bankia plummeted on reports, later denied, that customers were pulling deposits out of the Spanish lender. Fears of a full-scale bank run in Greece have not yet materialised. But the possibility of a deposit run in Europe's peripheral states is still very much alive. It is also the thing that policymakers are least prepared for. As with most aspects to the euro crisis, the usual answers are not much help. One tactic is to show customers the money. Old hands of emerging-market bank runs talk of how they used to pile cash up in full view of panicking customers so that they could see how well stocked the banks were with money. The equivalent now is to let the central bank provide enough liquidity that the ATMs always spit out cash. But if the idea is to get your hands on euros today in case of a currency redenomination tomorrow, then you will still want it out of the bank and under the mattress. Another response to runs is to calm worries about the solvency of specific institutions by beefing up the scale of deposit guarantees. In the first phase of the crisis, which now seems almost innocent in its simplicity, that is what governments did. But that makes the problem worse, not better, if government solvency is at the root of the problem. The logical solution, as we argue this week, is to set up a joint deposit-guarantee scheme, in which euro-zone states pool resources to provide credible reassurance that depositors across the zone will get their money back, up to a harmonised threshold of €100,000 ($125,000). To get around the redenomination risk, the guarantee would have to be a promise to repay the original value of the deposit in euros.

The problem, as analysts have noted this week, is that even if the political will to realise this end existed (which is highly questionable), it would take a long time to negotiate an agreement. There are all sorts of fiddly details for Eurocrats to get their teeth into. Should the scheme be prefunded? Should depositors be preferred creditors, or behind the ECB in the queue? What supervisory arrangements are needed to ensure that creditor nations have sufficient oversight of the deposit-taking institutions they now insure in peripheral countries? And that is before you get into the rigmarole of ratifying agreements.

The trouble with this is that there is a horrible, insoluble mismatch between the timescales to which Europe's policymakers work and the timescale of a bank run. A run is most likely within the next few weeks. And if a run starts, Europe's governments will have to reassure within a matter of hours. You might just about get a communiqué from Brussels in that timeframe, but could it really reassure when so many questions are unanswered?

If it does not, then the run will continue until such time as the banks close their doors to further withdrawals or the central banks have satisfied depositors' demand for cash. The former means trapping depositors inside a system they do not trust. The latter means providing liquidity to a banking system that has been abandoned by its own citizens. It would be hard to come back from either position.