The City is scared that Mr Darling will use today’s Pre-Budget Report to announce such a one-off levy. And high earners have rushed to make contingency plans with their employers, lawyers and accountants. Schemes to reduce a heavy tax bill include:

• increasing a banker’s basic salary in return for reducing the bonus element of their pay;

• beefing up benefits, which could include paying school fees;

• reclassifying a banker as a “consultant” thereby making him or her a self-employed businessman and escaping a banker tax;

• reclassifying a banker as being based in another country — quickly;

• splitting off trading desks and turning them into mini hedge funds so that those involved are no longer bankers but hedge fund managers.

It is believed that one Asian bank operating in the City has already implemented a plan where new bankers have been allocated at least two years’ worth of bonuses in the past few weeks, so they effectively squeeze under the wire of anything Mr Darling announces today.

One source said: “This is why a bonus tax is such a daft idea. Regardless of morality, such a tax would be totally ineffective because bankers will find endless ways of avoiding it.”