Any interest rate increase must trigger immediate benefits for customers, moneysupermarket.com has told Britain's high street banks.
It claims the banks stand to earn £240 million if they fail to pass on the rise rapidly.
Each rate rise of half a per cent nets £12 million daily for Britain's banks, the financial advice site estimates, although interest rates have always risen to date at the more gradual rate of 0.25 per cent.
But on the evidence of the last three rate rises, "it takes providers an average of 20 days to pass on an interest rate rise", explained head of savings and current accounts Kevin Mountford.
In the intervening period, the money mounts up.
In the event of a May rate rise, banks who cannot immediately pass on the benefits to customers should commit to backdating the effects thereafter, Kevin Mountford counselled.
One bank picked out for praise, however, is the Bank of Cyprus, which passed on rate rises the same day it heard the Bank of England's decision in August, November and January.
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