The Nationwide has criticised some banks and building societies for keeping customers in the dark when special savings deals are about to end.
The Nationwide has called for more clarity when special deals end
The UK's biggest building society said many customers were lured by high rates of interest which put savings accounts at the top of "best buy" tables.
But the Nationwide said many customers were not told when the initial rate of interest was about to drop.
It said all customers should be told in advance when initial rates were cut.
Matthew Carter of the Nationwide said it made no sense for customers to constantly switch their savings accounts.
"Some providers seem more interested in boosting profit and achieving best buy status than actually offering long term good value," he said.
"With many savings accounts, with introductory deals launched last year, savers could soon be languishing in accounts paying lower rates having enjoyed potentially high returns," he added.
The issue is not new.
For many years financial services organisations have used special promotions to put their accounts and mortgages temporarily at the top of the best buy tables.
These are widely published in weekend newspapers and some price comparison websites.
The main provider to the media of this comparative information is the financial information service Moneyfacts.
Its spokesman Andrew Hagger welcomed the Nationwide's idea of a code of conduct which would govern banks and building societies and which might bring some transparency to the matter.
"If you were a customer with a short term deal who received a letter warning you it was about to end and inviting you to consider another attractive deal, your relationship with the bank or building society is likely to last much longer," he said.
"Ironically, it's possible that some banks actually rely on customer apathy to fund these preferential short term deals, so if they had to notify customers when they ended, it may mean they are less likely to offer such deals," he added.
However a spokesman for the British Bankers Association (BBA) said the end of a special deal should not necessarily come as a shock to customers.
"The banking code says that at the outset of a special deal the customers should be provided with the maturity date as well," he said.