Nearly one million people could have their homes repossessed because they have no plan to pay off their interest-only mortgages, Citizens Advice has warned.
The charity said "time is running out" for an estimated 934,000 homeowners after research found they had made no arrangements to pay off their debt when their mortgage term ends.
They would be forced to sell their properties or could risk having their homes repossessed unless they find the necessary funds, it added.
The new figure is much higher than previous estimates from the Financial Conduct Authority (FCA) which said around 260,000 homeowners had no strategy to pay off their mortgages in 2013.
Rules were tightened in 2012 to ensure interest-only mortgages were no longer offered without a repayment plan, resulting in a major drop in the numbers sold.
But due to previous peaks in sales of the home loans - sold to millions in the 1980s and 1990s - the FCA has predicted waves of potential repossessions in 2017/18, 2027/28 and in 2032.
Citizens Advice chief executive Gillian Guy said interest-only mortgages had forced many people into a "financial black hole".
She said: "It is good that rules around these mortgages have changed, but there are many people who previously took out these products and face losing their home.
"Lenders have to exhaust all other options when borrowers get into arrears. It's time to level the playing field so that interest-only customers get the same protections when their mortgages mature.
"It is also important that people can get independent advice, guidance and support about how they can plan and manage their finances."
Some 3.3 million homeowners in the UK have interest-only mortgages, Citizens Advice said.
Those with the loans have lower monthly repayments than with a traditional mortgage because only the interest, rather than any capital, is paid off. However the full debt must be cleared at the end of their mortgage term.
According to Citizens Advice, some 934,000 homeowners with interest-only mortgages have no plan for repayment, while more than 432,000 have not even thought about how they will repay the capital.
In 2013 the FCA called on banks to contact all borrowers with interest-only mortgages due to end before 2020 to discuss how they plan to repay, but only 30% of borrowers responded.
Citizens Advice said people who already have the loans need more support because they do have the same protections as other mortgage holders if they fall into arrears.
Lenders should have to do more preventative work to help customers prepare for the end of their mortgage, such as phoning customers or offering face-to-face meetings, it added.
The charity said lenders should have to follow steps before taking an interest-only borrower to court, including:
:: A requirement to discuss a range of repayment options.
:: A requirement to discuss moving to an alternative mortgage product, including extending the duration of the mortgage.
:: Allowing enough time to sell their home for market price if necessary.
The research by Citizens Advice was based on a YouGov poll of more 2,100 homeowners.