A consumer group is urging savers to consider alternatives to instant-access ISA accounts offered by banks, slamming a "staggering" number of rate cuts by "scissor-happy" providers.
Which? looked at 21 providers and the rates of return they had offered on more than 200 products over the six years to April.
It (Other OTC: ITGL - news) said consumers - often the most loyal to a bank - were too often "wooed by a great rate" that quickly disappeared, with companies often offering returns below the Bank of England base rate, which has remained at 0.5% since March 2009.
The analysis identified rates as low as 0.05%, with customers often failing to switch providers once introductory rates had expired.
It said Halifax, Lloyds and Santander had all offered accounts with short-term rates to lure customers but the returns on those accounts had since fallen as low as 0.25%.
Which? said NatWest was the worst offender on cuts-per-account - with eight rate trims on two accounts over the six-year period.
It gave an example that if a saver had put the maximum amount allowed into a Natwest e-ISA in 2010, it would have earned 2%, or £204, in a year.
Six years later, the rate is 0.25%, Which? said, earning £25.50.
Harry Rose, Which? money editor, said: "Many savers simply want a provider they can trust to keep their Isa rate competitive.
"Too many banks are paying truly woeful rates of interest or are scissor-happy when it comes to cutting rates often penalising their most loyal customers.
"Our research shows savers who don't want to have to keep moving their savings about should consider parking their cash with one of the more reliable building societies who have been better at not cutting their rates for existing savers."
A spokesman for the British Bankers' Association, which represents the industry, said the Which? findings showed that consumers should "shop around for the best deal for them".
He said: "These have been frustrating years for savers.
"The Bank of England’s base rate has remained at a record low for several years and whilst this has been good news for borrowers it has fostered a low interest rate environment which has not been easy for many savers to bear.
"During this period banks have made it easier for savers to find the right products for their needs.
"The electronic Cash ISA Transfer Service has helped to significantly reduce switching times, interest rate disclosure on savings accounts has improved and a number of providers have streamlined their savings ranges to help customers to navigate the market.