Too many Isa providers are "scissor-happy" when it comes to chopping savers' rates, according to Which?
The consumer group analysed 212 instant-access cash Isas from 21 banks and building societies to find the "worst offenders" when it comes to rate cuts over the past six years. It looked at how many cuts were made as well as the number of cuts per account.
The research carried out in April focused on cuts for existing Isa customers and excluded cuts they would have known about when they took out the account, such as bonus rates expiring.
Which? said NatWest had the highest number of cuts per account of the providers it looked at, with eight cuts across two accounts over six years - giving it a rate of four cuts per account.
Its e-Isa previously earned savers 2% - but customers who haven't moved their money would now be earning a "meagre" 0.25%, Which? said.
The consumer group also found Tesco Bank had made three rate cuts on one account, while Royal Bank of Scotland had made two cuts on one account and Barclays had made 13 cuts across seven accounts.
Which? also found evidence to suggest some building societies had tended to make fewer cuts per account than banks in recent years.
Principality Building Society came out top of the research, having made just one cut across five Isas, and West Bromwich continued to pay existing customers a 1.25% to 1.55% rate they signed up to, even though the accounts in question had closed to new business. Which? said Coventry Building Society also deserves credit for rewarding existing customers, who earn upwards of 1.5% on closed accounts.
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."
Which? has tips for finding the best cash Isas at www.which.co.uk/money/savings-and-investments/guides/how-to-find-the-best-cash-isa.
A spokesman for the British Bankers' Association (BBA) 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.
"The introduction of the new personal savings allowance from this April means that most savers will no longer have to pay any tax on their cash savings. We always encourage customers to review their savings regularly and to shop around for the best deal for them."
A NatWest spokesman said: "We've simplified our savings products to help meet our customers' needs. In addition we no longer offer teaser rates meaning existing customers benefit from the same or better savings rates than new customers."