The study found 1.1m interest-only mortgage holders either had no investment plan for capital repayment or one that is not on track to clear their debt.
However, just over one in ten of those with an interest-only mortgage said they intended to repay the capital by selling their home at the end of their mortgage term.
BDRC Continental director Tony Wornell said: “Our research suggests that some interest-only borrowers are not engaged with the end game – what will happen when their mortgage term finishes and they have to repay the capital.
“Everyone with an interest-only mortgage needs a credible repayment plan.”
Among those with no investment plan, almost a quarter expected to switch to a repayment mortgage, while 16% considered selling the property or using cash savings to repay the capital. A quarter said they did not know how they would repay the capital.
Options open to those falling behind with their investment plan included using an endowment policy, cash savings, switching to a repayment mortgage or selling the property.
Age Partnership equity release technical manager Simon Chalk said he had seen a “spurt” of interest-only mortgage holders turning to equity release since the financial crisis and particularly since 2011: “It is not nice for people to find themselves in that position but part of it is poor planning and part of it is poor understanding of the mortgage they have taken out.
“It is absolutely critical that every single mortgage lender offers independent equity release advice.”