It is six months since the increased ISA limits were universally introduced, but millions are still yet to take advantage.
There are more than 17.5 million cash ISA holders in the UK, with only the ubiquitous current account more widespread amongst consumers.
But research conducted by Halifax has revealed that most ISA customers are not taking advantage of the enlarged limits.
From April 6 this year, savers have seen their 2010/11 cash ISA allowance increase from £3,600 to £5,100.
Just over a fifth (22%) of ISA customers have topped up their account this year, however, with an average inflow of only £1,400, some £3,700 less than their yearly limit.
Should they decide not to add further funds to their accounts, savers could be missing out on £49 million of valuable tax-efficient interest.
"If you have something to save, no matter how large or small an amount, it should be saved in an ISA," Russell Galley, managing director of savings at Halifax, said.
"There are ISAs to suit every type of saver and we're concerned that so many people are failing to use these valuable savings vehicles to their maximum.
"We're also worried about those savers with an ISA who are not making the most of it either by failing to top it up or languishing in a poor rate when they could transfer for a better return."
Despite the possibility of their accounts now paying uncompetitive rates of interest, many ISA customers are reluctant to transfer their savings to a better account.
Losing out on interest, the process being too much hassle and not even knowing that switching is allowed are cited as reasons for leaving funds where they are.
However, new rules mean providers must ensure that clients' balances are switched within a 15 day timeframe.
For those that would like the process to be quicker, there are other options.
Halifax is aiming to make the cash ISA transfer system more transparent for customers.
The first bank to offer such a promise, the provider is offering customers instant interest on cash ISA transfers and other institutions are likely to follow.
The cash ISA promise includes several other benefits: Halifax cash ISAs will be available to both new and existing customers and the interest rate will be clearly stated on ISA statements and online.
In addition, Halifax cash ISA customers will receive advanced notice when any reward or fixed rate period is due to end.
The pick of its ISA offerings is the Cash ISA Direct Reward, offering clients a top rate of 2.80%.
Other providers that have made ISA promises include Santander, which says it backdates interest from the date the transfer leaves the existing ISA, not the date it is received.
In addition, Barclays also says it provides a transparent service for its ISA customers sending them an update two months before their bonus rate ends.
For the best ISAs the market has to offer, check the Moneyfacts.co.uk Best Buy tables, and celebrate the six month anniversary of the enlarged limits with a tax efficient account.
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Disclaimer: Information is correct as of the date of publication (shown at the top of this article). Any products featured may be withdrawn by their provider or changed at any time.
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