Savers chasing the top cash rates are being thwarted by poor service and hidden catches on some of the market’s “best buy” accounts. ICICI, the internet savings provider that boasts a one-year bond fixed at 7.2%, the best savings rate available, has been the subject of a raft of complaints about technical problems.

This comes amid widespread problems for cash Isa savers, with many big brand names unable to cope with demand. Other cash savings providers have been criticised for publishing misleading headline rates.

ICICI customers have had difficulties logging on and transferring funds. Some complained that they were unable to access their accounts for more than 24 hours on at least three occasions in the past six weeks. The Indian-based bank, which also offers an easy-access savings rate of 6.16%, confirmed computer and “software-related incidents” had caused difficulties for customers in recent months.

Dan Plant, of Moneysavingexpert, the consumer website, said: “In the last few weeks we’ve seen a rise in complaints relating to service issues with ICICI. There is high demand for top-paying accounts and savers need to be aware that this can lead to administrative errors and poor service.”

Sophie Gist, 32, of Brighton, East Sussex, was unable to access her account online this month. “Thankfully it wasn’t urgent but it was very worrying,” she said. “I put my username and password in and the screen was just blank. There have been two or three times when I haven’t been able to log on.” She also has an account with Icesave, part of Iceland’s Landsbanki, and said she would switch all her savings there if she had any more problems.

Meanwhile, savers wanting to transfer cash Isa money have been forced to wait months for accounts to be set up. Providers including Nationwide and Barclays have struggled with huge demand as consumers retreat to the relative safety of cash.

Last week the industry promised to speed the Isa transfer process after Vince Cable, the Liberal Democrat Treasury spokesman, threatened to refer providers to the Competition Commission.

Birmingham Midshires is also criticised after gaining a place in the best-buy tables with an internet account that offers an annual equivalent rate of 6.52%. Savers must withdraw the interest accrued each month or transfer it to another account.

Andrew Hagger, at Moneynet, the comparison site, said: “The interest does not compound each month, so the highest you can achieve on savings is the gross rate of 6.33%. There’s something fundamentally wrong here.”