The bank indicated it would defend this level unless oil prices fell to $30 a barrel. With Urals blend still around $43, the rouble's value currently looks in the right ballpark. The trouble is that currencies tend to overshoot as they decline. Forward markets are pricing the rouble 18 per cent lower against the dollar in 12 months. And the bank cannot afford indefinitely to keep burning through its remaining $386bn foreign exchange reserves, which have fallen since August by nearly $10bn a week. The pressure on its reserves was highlighted by Fitch's downgrade on Wednesday of Russian foreign and local currency ratings to two notches above junk, in line with a similar move by Standard & Poor's in December.

Raising interest rates might work, but only a hefty hike of, say, 500 basis points. The relatively underdeveloped banking system would only partly mitigate the blow to the weakened economy. The remaining options are capital controls – or letting the rouble float. But the political costs of a further devaluation, given Russians' obsession with the rouble/dollar rate, would be enormous. Russia has also burned through $200bn in reserves to get to this point. That makes Wednesday's one-off 20 per cent devaluation of neighbouring Kazakhstan's tenge look preferable to Russia's protracted depreciation.

But Kazakhstan's move may still prove insufficient. With capital inflows into emerging Europe plunging, Hungary's forint has hit a record low, the Romanian leu is close to one, and the Polish zloty is at its weakest since 2004. Eastern Europe risks tipping into a regional financial crisis with knock-on effects across the continent.