Savers have been warned they could be missing out on as much as £242 million of tax-free interest because they leave spare cash sitting in their current accounts rather than putting it into an ISA.
In a typical month, the average cash ISA holder has £498 left over in their current account as the next payday arrives, according to Santander Savings.
Yet despite being in a position to enjoy a tax-free return on this money, only 17% will take full advantage of this year's £5,340 cash ISA allowance.
"Maximising the return on spare cash is important and taking advantage of tax-free interest via a cash ISA is a great place to start," said Matt Hall, head of savings at Santander.
"With only a few days to go to the end of the tax year, I'd urge savers to make use of their ISA allowance immediately as any unused allowance is lost forever."
Meanwhile, anyone with a cash ISA that includes a bonus rate which is set to come to an end has been urged to move their money to a better rate account.
Based on the current cash ISA limit of £5,340, Which? estimates those people who do not switch accounts once the bonus period expires will lose an average of £85 in the coming year.
The consumer watchdog said there are some clear incentives to switching as many banks have increased their top paying interest rates for 2012.
However, before transferring, consumers have been urged to compare cash ISA accounts and work out which provider will offer them the best rate.
If the best-paying accounts come with a big bonus, savers should then note when the bonus expires and switch before their interest rate, and potential earnings, drop.
For anyone who has still not made use of their 2011/12 ISA allowance, the 5 April deadline is fast approaching.
"If there's only one thing that people do about their finances this week, it should be to look at whether they would be better off by moving their money to a new account," said Which? executive director, Richard Lloyd.
Download our free Beginner's Guide to ISAs to find out how to use your 2011/12 ISA allowance. You will need a PDF reader such as Adobe Reader to download this guide.
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