Greece faces the threat of state bankruptcy. No longer is there any illusion that membership of Europe's economic and monetary union (Emu) provides protection from harsh realities. Since it entered the euro area in 2001, Greece has sacrificed competitiveness and amassed enormous trade deficits. Theoretically, to make up the economic ground lost in less than a decade, the Greeks would need to devalue by 40 per cent. But in a monetary union, that is impossible.

There is no shortage of proposals to help the Greeks, including assistance from other Emu governments – a move that would contravene the “no bail-out” rule enshrined in the treaty setting up Emu. There is, sadly, only one way to escape from this vicious circle. The Greeks will have to leave Emu, recreate the drachma, and re-enter the still-existing exchange rate mechanism of the European Monetary System, the so-called ERM-II, from which they departed in 2001.

We do not make this statement lightly. As the four professors who took the German government to the German constitutional court in 1998 over Germany's entry into the euro, we have a track record of straight-thinking and plain-speaking. In a landmark judgment in 1993, the constitutional court ruled that, once it came into force, Emu had continuously to satisfy the full conditions of the “stabilisation treaty” concluded when the single currency was agreed. If it did not, the court ruled, Germany would be obliged to leave Emu.