Thousands of mortgage borrowers could get a cut of compensation if a High Court trial rules in their favour.
Homeowners who held mortgages with TSB bank in 2008 have been described as being "mortgage prisoners" by law firm Harcus Parker.
The law firm says this is because TSB put borrowers on its Whistletree brand paying interest rates as high nine per cent after Northern Rock collapsed in 2008.
It came after TSB bought £3.3billion of Northern Rock mortgages which covered around 27,000 homeowners in 2015 under its Whistletree brand.
Lawyers at the firm Harcus Parker are working to take action on behalf of customers who believed they were unfairly charged high mortgage rates.
Damon Parker, senior partner at Harcus Parker, told the Sun that the law firm had a High Court Trial against TSB and urged former and current TSB customers to sign up for the claim "fast".
This is because legal time limitations can reduce the amount of compensation they receive.
Damon told the Sun: "The fact that we will now have a trial to decide if TSB has breached the terms of my clients’ mortgage contracts is very positive news.
"It means that the whole process can be expedited.
"We are confident in our contention that Whistletree customers were unfairly trapped on high-interest rates and that they should be repaid back the extra money TSB unfairly charged them.
"We hope that it will set a legal precedent for all mortgage prisoners who have endured financial misery through no fault of their own.
"While these people have suffered, large institutions have made hundreds of millions of pounds."
"We do not want mortgage prisoners to wait and see what happens because TSB will argue that every day they delay starting their claim will reduce the final amount of money that they can recoup.
"It’s really important if you think you have a claim to join up as soon as possible."
Damon told the newspaper that the law firm is offering people help on a "no win no fee" basis.
Harcus Parker has previously said there could be more than 200,000 "mortgage prisoners", so have been stuck on high interest rates after their loans were sold, overall and have frequently urged people to come forward with their claims.
It has set up a website - mortgageprisonersclaims.com - specifically for mortgage holders who believe they may be entitled to make a claim following the sale of the Northern Rock mortgage book.
A two-day High Court hearing is expected to begin next year.
A TSB spokesman said: "Whistletree customers are not mortgage prisoners.
"Since we took over the management of these mortgages, over two-thirds of Whistletree customers have either moved to a new mortgage or closed their mortgage with Whistletree.
"We write to every customer, twice a year, to remind them that they can switch."
