Alistair Darling has a presentation problem. The chancellor will on Tuesday proclaim the restructuring of RBS and Lloyds – the two part-nationalised banks – as good news for the taxpayer: a sign that the worst of the banking crisis is over.

But at the same time he has the awkward task of explaining why, if that is the case, he is about to put billions more pounds of public money on the line by injecting new capital into the two banking giants.