Currently, £11,280 can be saved in a stocks and shares Isa while only £5,680 can be saved in a cash Isa.
Research by Nationwide and Yougov of over 2,000 people found the differences in Isa rates and accounts made it confusing for consumers.
Some 30 per cent found the rules confusing and 23 per cent said the rules were putting them off saving into an Isa.
Over 61 per cent save into a cash Isa rather than a stocks and shares Isa and 81 per cent did not know how much you could save in a stocks and shares Isa.
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Some 26 per cent said the difference in limits was 'unfair' and 22 per cent said they would be more likely to use a stocks and shares Isa if the limits were equalised.
Bringing the limits into line with each other would help first-time buyers, pensioners and make the system simpler, the firm said.
Richard Marriot, Nationwide's head of savings, said: "Isas are clearly popular and an extremely useful way of saving tax-free, but it seems a significant proportion of people are confused by the current rules. This could mean individuals are less likely to use them to save.
"Making the limits the same for both cash and stocks and shares Isas would help to simplify them, giving savers more confidence about opening and getting the most out of their Isa.
"Calling for parity on Isa limits and the ability to transfer from a stocks and shares Isa to a cash Isa is just one of the ways in which we are working to meet our target of empowering one million people to start saving by 2017."

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