Banks have been accused of penalising loyal customers by paying them significantly lower interest rates than new customers.
Long-standing account holders with balances of £20,000 are missing out on up to £292 a year, analyst Savings Champion warned.
Instead of increasing rates for all savers, some banks are releasing new versions - or “issues” - of easy-access accounts which offer higher returns. However, existing customers do not automatically benefit from the new, higher account rate.
Three of Britain’s 10 biggest banks and building societies - Nationwide, TSB and Santander - are offering their loyal customers a worse deal than new customers, the research found.
Nationwide pays 1.75pc on the 1 Year Triple Access Online Saver 15 account, compared with 1pc on some older versions. At TSB, savers earn 0.25pc on the Easy Saver and eSavings accounts, but 0.2pc on some previous versions.
Santander has moved customers with older eSaver accounts to the Everyday Saver, which is at 0.1pc, instead of upgrading them to the new issue rate of 0.75pc.
Out of the top 25 easy-access accounts paying the highest rates, 14 have older versions paying different rates. Seven of the 13 banks offering these accounts have not upgraded the interest rate on older accounts.
Anna Bowes, of Savings Champion, warned customers with older accounts that they may not be earning the rate that new customers do. The Bank of England has raised interest rates by 1.65 percentage points since December and further increases are expected.
Ms Bowes said: “I think some of the providers are hoping that customers aren't active and don't realise what they're earning.”
She recommended that customers either switch internally to the new version of the account or look for a more competitive rate at another bank.
Sarah Coles, of the investment broker Hargreaves Lansdown, said the “loyalty penalty” was a major issue. “There are no rules that force banks to offer all savers the same rate when rates rise, so it’s fairly common to bring in new issues of an account, with a better rate,” she said.
Nationwide, TSB and Santander said their customers could easily move their money into new accounts and had been told about changes in savings rates. They said many account account holders had transferred their funds.