Savers are losing out on almost £1 billion by not putting money into their cash Isa accounts, Barclays has warned.
Around 40% of people with cash Isa accounts have not used it to deposit any savings in the current tax year, the bank's research showed.
The average saver with a cash Isa has accrued £42,000, more than double that of those without, Barclays said.
While more than 21 million Britons have a cash Isa account, less than a fifth (16%) take advantage by using their full limit of £15,240 in the current tax year, the research suggested.
The bank is reminding savers they can still take advantage of the allowance for this year if they pay the money in before April 5, the end of the tax year.
More than three quarters of those with cash Isa accounts were found to be unaware the money they put into their account goes towards the annual allowance even if it is then withdrawn, Barclays said.
Clare Francis, savings and investing expert at Barclays, said a lack of awareness about how the allowance works is seeing savers miss out.
"While it's encouraging that so many people are able to set aside money for savings, the relatively low number using their full Isa allowance suggests they may not be making their savings work as hard as they could.
"Average savings are higher than the amount needed to fill an Isa, but it appears low levels of awareness around the allowance and how it works could be leaving savers needlessly missing out.
"However, the good news for those who have not used their full Isa allowance yet this year, is that it's not too late.
"They have until 5 April to make use of the 2015/16 allowance and then from 6 April, they will have a new £15,240 Isa allowance which can be used during the 2016/17 tax year."
:: The research for Barclays was conducted by Opinium between February 19 and 23 using a nationally representative sample of 2,005 UK adults.