Research by the country's second largest mortgage lender, shows that more than one in three borrowers (37 per cent) are not saving to repay the amount borrowed on their interest only mortgage.
The bank points out that while rising housing prices have seen many people take out interest-only mortgages, if rates fall selling the house could recoup less than the original loan leaving those without savings in place struggling.
Abbey reports that three mortgage holders in ten have interest only policies, and more than a third of these have no savings in place to repay the value of the loan.
One in four of those not saving to repay their interest-only mortgages do not know how they will pay back the amount borrowed, with 20 per cent more saying they are relying on their property to increase in value to cover the loan.
Gary Hockey-Morley, Abbey's director of mortgages, commented: "I am surprised by the number of people leaving things to chance. Every mortgage is going to have to be paid back eventually and borrowers should never lose sight of that.
"People should not go into an interest-only mortgage without a plan to repay, even if it means saving just small amounts to start with. Borrowers who haven't made plans should get in touch with their mortgage provider to seek advice as soon as possible."
Of the people with savings in place, a large number of readily admit that they will not have enough to cover the amount of the original loan.
In Scotland 43 per cent of those with interest only mortgages and savings in place admit the money they are putting away will not be enough to repay the mortgage in full, with a further 29 per cent not sure if their savings will be adequate.
Moreover 80 per cent of interest-only borrowers in Scotland are first-time buyers, compared with a national average of 57 per cent.
Nationally, of those with savings one in four have an endowment policy, another ten per cent have a combination of endowments and other savings, 18 per cent do not know what their savings are, and over 11 per cent have savings accounts such as ISAs.