The majority of banks are failing to inform customers about changes to the interest they are earning on their cash, a consumer group said today.
Which? Money found that only four of the 12 banks and building societies it investigated guaranteed that they would personally inform their customers about savings rate changes.
The other groups relied largely on people seeing adverts in newspapers, or visiting their branches, to find about about interest rate cuts or rises.
The four providers that promised to inform their customers of any interest rate cuts by letter or email were Cheltenham & Gloucester, First Direct, the Co-operative Bank, and ING Direct.
Other banks said they would personally notify customers only if the rate they were receiving was cut by more than 0.25%, or if a series of smaller cuts added up to more than 0.5% during a year.
But Which? Money said with the Bank of England base rate currently at a record low of 0.5%, this policy could lead to customers not being notified about rate cuts that were "proportionately very large".
Peter Vicary-Smith, chief executive of Which?, said: "Our rigorous research shows that outdated and inconvenient methods of notice on interest rate changes are keeping savers in the dark for longer, at a time when they need greater disclosure than ever before.
"This is just another example of banks treating their customers badly. As our latest savings satisfaction survey shows, once again it's the smaller players that offer better service, and have happier customers."
The group also warned that the proportion of mortgages and regular savings accounts that were available only to people who had another product with the same provider had "increased dramatically" since the beginning of the year.