TSB faces a High Court case that could lead to thousands of borrowers receiving compensation.
Lawyers say 27,000 customers of collapsed Northern Rock were trapped on high interest rates when the bank bought their mortgages in 2015.
TSB has agreed to a court hearing next year, to let a judge rule whether its Whistletree mortgage holders were exploited.
More than 6,500 are represented by law firm Harcus Parker, where partner Damon Parker said: "While these people have suffered, large institutions have made hundreds of millions. That cannot be right."
Ainsley Donnelly, 49, claims he had to pay 6% interest before being moved to 2.74% in 2020. He said: "It's been pretty scary sometimes.
There have been occasions where I just haven't had petrol to get to work and I've had to borrow money from my family. I feel pretty angry."
A TSB spokesman said: "We write to every customer, twice a year, to remind them they can switch."
