Research from the firm also reveals that many are unsure whether or not to take out an ISA this financial year.
Significant gains can be made for customers willing to fill their ISA allowances with a single "lump sum" payment, analysis from one provider has shown.
Abbey advised savers to invest their entire £3,600 cash ISA allowance this month, in order to benefit fully from compound interest. The firm pointed out that savings built gradually throughout the year would generate less of a return.
According to Abbey, the average amount saved into a cash ISA last year was £2,082 - barely half of the maximum allowance. Pressure on budgets from the credit crunch and low interest rates from the Bank of England are thought to have contributed to this shortfall.
The provider also revealed that 28 percent of investors were still "undecided" as to whether or not they should take out an ISA at all this year - let alone maximise their £3,600 allowance.
Reza Attar-Zadeh, director of savings & investments at Abbey, said: "In the current climate, every penny counts so making your savings work hard for you all year round is crucial. A cash ISA is a great way to make sure the taxman doesn't get his hands on the interest you get on your hard earned money."
He added: "The new tax year is now here and the sooner an ISA is funded, the greater the return on the money saved."
The new 2008/09 financial year began on Monday.
