Nearly a million people are in danger of having their homes repossessed because they have interest-only mortgages they can't pay off.
With these, the borrower only pays interest, rather than paying off the loan itself in instalments as well. At the end of the term, the original sum becomes due - and if the borrower can't find the cash, the house will have to be sold or even repossessed.
Many people have a separate payment plan in place to cover this - but according to Citizens Advice, 934,000 don't. The figure is much higher than previous estimates from the Financial Conduct Authority (FCA), and from lenders themselves.
"People buy a home for stability – but interest-only mortgages have forced many into a financial black hole," says Citizens Advice chief executive Gillian Guy.
"It is good rules around these mortgages have changed, but there are many people who previously took out these products and face losing their home."
The charity says that in the UK there are 3.3 million mortgage holders who have interest-only products. Of these, the survey shows that around 1.7 million have no linked repayment vehicle such as an endowment or ISA.
Shockingly, the survey revealed that many interest-only borrowers claim not to have been made aware that they would need to repay the capital at the end of their term.
Rules were tightened in 2012 to make sure that interest-only mortgages were no longer available without a repayment plan, but Citizens Advice says existing customers need more support.
And interest-only mortgage holders don't have the same protections when their term ends as other mortgage holders do when they fall into arrears.
While lenders now have a legal obligation to consider alternative options before starting possession action - including extending the length of a mortgage, changing its type and giving people reasonable time to sell up - these protections do not apply to interest-only mortgages at the end of the term.
"Lenders have to exhaust all other options when borrowers get into arrears," says Guy.
"It's time to level the playing field so that interest-only customers get the same protections when their mortgages mature."
In 2013 the FCA called on banks to contact all borrowers with interest-only mortgages ending before 2020 about how they plan to repay. However, only around 30% of borrowers responded.
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