About a quarter of Isa holders are unaware of the new Isa limits that will increase their total annual allowance to £15,000 in July, research suggests.
Some 23% were ignorant of the changes to Isas arising from the Budget in March, which included the higher limit.
The ability to switch funds between stocks and shares Isas and cash Isas was also introduced by George Osborne.
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The Halifax survey found 32% of cash Isa holders expected to save more as a result of the Isa reforms, while 52% believed it will make no difference to their saving habits.
While 61% of people surveyed were able to identify some of the financial benefits in the new rules, such as the new higher limits, 28% would like more help in understanding the changes.
Knowledge of the changes was lowest among younger people. Some 40% of people between 18-24 and 36% of 25-36 year olds were unaware of the changes to the Isa rules.
Richard Fearon, head of Halifax savings, said: "The new Isa rules have the potential to transform the level of UK's savings. However, in the short term there remains confusion about how much people can save in their Isa and when.
"As a result, many savers could already be missing out on tax-free savings by being unaware that it's possible to start saving now and top up to the full limit when it becomes available in July."