There has been a big jump in the number of people complaining about the mis-selling of endowment mortgages.
Millions of homeowners are facing endowment shortfalls
Complaints to the Financial Ombudsman Service about endowments rose to 51,917, up from 13,570 on the previous year, the service's annual report said.
Millions are facing shortfalls on their endowments, an investment product sold heavily in the 1980s.
It is estimated that 3.5 million households are now facing shortfalls of more than £5,000 on their loans.
Many borrowers were told their endowments were guaranteed to pay off their mortgages, but some of those promises have fallen well short.
Consumer groups are concerned that many people are ignoring warnings about their endowment - and are also unaware they may be eligible to claim compensation for mis-selling.
In particular, there are strict rules surrounding the timing of complaints.
An estimated 700,000 people may have already left it too late to complain about their mortgage endowment. The general rule is that people must complain within three years of receiving their first "red letter" from their insurance company or lender.
The rise in endowment complaints accounted for a large part of the overall rise reported by the ombudsman.
In total, complaints made to the ombudsman were up by 57% on the previous year.