Moneyfacts.co.uk said the looming personal savings allowance, which from April 6 will mean many people no longer have to pay any tax on their savings held outside Isas, may be partly to blame for making Isa rates worse.
In the past, Isa providers have made their offerings more attractive around this time of year, to woo savers who have yet to use their Isa allowance of £15,240 before the current tax year ends, or who are looking for a place to stash their cash when the new tax year starts from April 6.
Isas currently have tax advantages over other accounts. But people's reliance on Isas as a shelter from the taxman is expected to be weakened from April 6, when the new personal savings allowance is introduced.
The change means basic rate taxpayers paying 20% tax will be able to earn up to £1,000 in savings interest tax-free and higher rate taxpayers paying 40% tax will be able to earn up to £500 in tax-free savings income.
The new allowance will include account interest from bank and building society accounts as well as accounts with credit unions and NS&I (National Savings and Investments) and other types of income, such as that from government or company bonds.
Interest earned from tax-free Isas will not count towards the personal savings allowance, as money held there is already ring-fenced from tax.
Moneyfacts said that five years ago, the top easy access Isa rate was 3.15%.
Six months ago, it was 1.51%, and now it is a deal at 1.41% being offered by Virgin Money's E-Isa. Also among the top current deals, Coventry Building Society and Post Office Money are both offering easy access deals at 1.4%.
Someone locking into a five-year fixed Isa five years ago could have got a rate of 4.3%.
But if they are looking for a similar deal now, the top rate currently on its records is 2.6%, from the State Bank of India, which requires a minimum £15,000 investment. Also among the best buys, Metro Bank has a five-year deal at a slightly lower rate of 2.25%, with a minimum £1 investment needed.
Charlotte Nelson, a finance expert at Moneyfacts.co.uk, advised savers to "act fast" if they see a good deal as it is likely to become over-subscribed quickly and have its rate reduced.
She said: "If we do have an Isa season this year it's likely to be short-lived with rates only rising slightly, if at all."
Research from Nationwide Building Society recently found that more than two-fifths (45%) of people are already using their current account to build up their savings pots, compared with one in three (34%) who use a cash Isa.
Several current account providers are offering rates as high as 3%, 4% and 5%, together with added perks such as cashback and cash for switching.
Ms Nelson said the new personal savings allowance "seems to have only exacerbated the downward slide in Isa rates".
She continued: "Isas were once the go-to product for savers as they offered not only tax benefits but also some of the better rates on the market.
"However, this is certainly no longer the case thanks to almost constant rate decreases. For instance, the best easy access Isa rate has dropped by 0.10% in just six months, making this perhaps the worst Isa season on record."
But Ms Nelson cautioned savers against forgetting Isas completely in the light of the new personal savings allowance.
She said: "Relying on the personal savings allowance alone for your tax-free pot is a gamble - eventually rates will go up, and the amount savers can save tax-free will subsequently diminish.
"For this reason, Isas shouldn't be overlooked, particularly if you have larger amounts to save. In addition, Isas can be passed on to spouses after death, which is worth contemplating when weighing up your long-term interests."