Two thirds of Brits have failed to open an Isa this tax year - missing out on billions of pounds in interest, according to TSB.
While, nearly a quarter of people (22%) have put away some cash, the average balance stands at just £1,211 - that's £4,549 short of this year's allowance limit.
Only 15% have used their entire tax-free limit of £5,760 for the current tax year, which ends at midnight on Saturday.
According to the bank, taking the average Isa rate of 1.64%, savers would be able to make a huge £3.5 billion in interest this year alone if they were to save the maximum in cash.
But in a sign that people are still finding it difficult to make ends meet, 63% said they didn't save into an Isa because they didn't have any spare cash at the end of the month to do so.
Meanwhile, one in five (20%) people said they didn't bother to save as they were put off by the low interest rates on the accounts.
Thinking long term
Jonathan Hall, head of savings at TSB, said: "Interest rates might be low at the moment, but thinking about the long-term health of finances is important.
"Taking advantage of tax-free savings now should be high on the priority list as it puts more money back into people's pocket. Even if someone has just a few pounds to save, they should be saving it in an Isa."
He added: "We know a healthy economy relies on people saving. At TSB every penny deposited is only used to help other TSB customers through mortgages and loans. The more people save, more loans can be made which helps local communities thrive right across Britain."