"I think it will be a one-off thing because, frankly, the very people you are after here are very good at getting out of these things."
Alistair said "imaginative ways of avoiding it in the future" were inevitable, The Financial Times reported Thursday.
The tax was set at 50 percent for bonus pay in excess of $38,500.
At a financial service conference in London, Darling said what he had intended to do "was to send a message to them (bankers) that we all live in the same world together."
Renewing the bonus tax is not among the new coalition government's budget proposals, but a $3 billion levy on banks is under discussion, the Financial Times said.