The radical measure will also mark a watershed in the Bank's history since it was handed independent control of interest rates by Gordon Brown nearly 12 years ago. Until recently, that was seen unquestionably as Mr Brown's masterstroke. On a bright morning on May 6, 1997, the then Chancellor announced that he was surrendering to the Bank his power to set base rates.

It was a landmark moment. In the following decade, the country grew used to the idea that it was no longer the Treasury, but the Bank, that was at the economy's helm. On one Thursday each month, the hand of the "Old Lady of Threadneedle Street" was felt on the tiller as the Bank's decisions emerged at midday.

Since the MPC has had to ask the Chancellor for permission, critics believe this undercuts its autonomy. Mr Darling will set a maximum amount of money to be created. Yet it will still be the nine MPC members who decide when to act and to what extent. It is a grey area, but unquestionably the Chancellor's hand is creeping back on to the steering wheel. An acid test will be whether the MPC can decide by itself when to stop.