The number of homeowners trapped on a standard variable rate (SVR) mortgage has rocketed by 40% since last year to reach more than 800,000, research has found.
There are around 839,000 of these borrowers, making up 7.4% of the total mortgage market, and they will be unable to switch to a better deal unless their finances improve or house prices rise, said HSBC.
The regions with the highest proportion of "mortgage prisoners" as a share of total mortgages are the North West (10.2%) and the North (9.9%).
The number of borrowers stuck with insufficient equity in their homes has soared by 39.6% since autumn 2011, which has been prompted by large volumes of homeowners coming off discounted or fixed-rate deals taken out since 2007 combined with house price falls, HSBC said.
Scotland has seen the fastest increase in the numbers of mortgage prisoners, with a 75% increase since autumn 2011, taking the total number to just under 92,000. This is largely due to the country coming off worse than some English regions in terms of recent house price falls, said the study.
A string of lenders have raised their SVRs since the spring, pushing up mortgage costs for more than a million homeowners. Lenders have also toughened their borrowing criteria amid the difficult economy, making it harder for people to switch to a cheaper deal.
But HSBC also calculated that around four out of five borrowers on an SVR, equating to 3.6 million people or a third of the total mortgage market, could potentially remortgage to a lower rate.
The study found that the average lender's SVR was currently 4.86%, and to be eligible for a better rate at below 4%, borrowers would need at least 15% of equity in their home.
It said that 3.6 million SVR borrowers had the sums of cash in their home to meet this criteria and a borrower with a £100,000 mortgage who got onto a deal with just 0.5% in interest savings could save £336 a year.
Wales was named as the area with the highest proportion of SVR borrowers who could potentially move their deal, at 38.8% of mortgage holders.