Instant access and cash Isa accounts have suffered around 20 rate cuts a month on average over the last year and a half, consumer group Which? has found.
The research found savings providers have hacked rates on these accounts 343 times since August 2012, despite the Bank of England base rate being stable during that time at 0.5%.
Which? found that rates have even been slashed at the height of "Isa season" - the period around the end of the tax year when providers would normally be expected to ramp up competition to attract people who are looking for somewhere to put their Isa allowance or that for the next tax year.
The Government's Funding for Lending scheme, introduced in August 2012, has been blamed for making savers' plight worse as it gave banks access to cheap funding to help mortgage borrowers, making them less reliant on having to attract savers' deposits. The scheme has been redirected away from households for 2014.
The consumer group also found that in February 2008, the average instant-access savings account paid a rate of 4.14%, but that has now plummeted to 0.63%.
This slide over the last six years would equate to an annual loss of £351 for someone saving £10,000 in an average instant-access savings account.
The research found that more than half of accounts fail to even beat the base rate. Of the 205 instant-access savings accounts available to new customers, 111 pay 0.5% or less.
Which? executive director Richard Lloyd said: "It's no secret that savers have been hit really hard in recent years, so it's important to be a savvy saver and take advantage of the best deals across the whole market, not just savings accounts.
"But banks and building societies should make it easier for savers by giving them clearer information. This includes putting interest rates on all statements, sending annual summaries, and better notification when a bonus rate ends or if better accounts are available."
The consumer group's tips to help people make the most of their savings include using all your tax-free Isa allowance, going online as seven of the 10 best rate instant-access accounts are only available on the internet, and thinking about saving in a current account.
TSB's new current account pays 5% on balances up to £2,000, Nationwide has a current account which pays 5% interest for the first 12 months, and Santander's 123 current account pays a rate of up to 3% plus cashback on bills.