The break-up of the euro would be a multi-trillion euro catastrophe. An interest subsidy costing around 50 billion euros over seven years could help save it.

The immediate problem is that Spain’s and Italy’s borrowing costs - 6.3 percent and 5.8 percent respectively for 10-year money - have reached a level where investors are losing confidence in the sustainability of the countries’ finances. A vicious spiral - involving capital flight, lack of investment and recession – is under way.

Ideally, this week’s euro summit would come up with a solution. The snag is that most of the popular ideas for cutting these countries’ borrowing costs have been blocked by Germany, the European Central Bank or both.

Take euro bonds, under which euro zone countries would collectively guarantee each others’ debts. They would allow weak countries to borrow more cheaply. But Germany won’t stand behind other countries’ borrowings unless they agree to a tight fiscal and political union which prevents them racking up excess debts in future. Such a loss of sovereignty France, for one, will find hard to swallow.

Or look at pleas for the ECB to buy Italian and Spanish government bonds in the market. That too would cut their borrowing costs – for a while. But when the bond-buying ends, the yields would just jump up again. Private creditors would merely use the opportunity to offload their bonds onto the public sector. The ECB has already spent 220 billion euros buying sovereign debt with no lasting impact, and is reluctant to do more.