A US private equity company has been accused of misleading the government about the biggest sale of state assets in UK history.
BBC Panorama has discovered Cerberus told the government it was planning to offer homeowners better mortgage deals before its £13bn purchase of former Northern Rock mortgages in 2016.
But the company hasn't provided any new mortgages and 65,000 homeowners are still trapped on high interest rates.
Cerberus denies the allegation.
Many of the homeowners are mortgage prisoners, who cannot shop around for a better deal because their loans are too large or their credit rating has been damaged.
Lisa and Mark Elkins have to pay £2,500 a month on their mortgage because their interest rate is nearly 5%. That's about three times the best market rate.
They have tried to keep up with the monthly payments by taking on extra jobs and working long hours. The couple has also borrowed £20,000 from family and friends.
But Lisa says they now have no choice but to sell their home of 15 years: "I love the neighbours and I love the house but it's become a rock around my neck. You feel like you're sinking and you can't get up and I can't have that any more."
The couple has been trapped on high interest rates since the government took over Northern Rock in 2007.
BBC Panorama has calculated that the high rates have cost the Elkins an additional £30,000 over the past decade.
Mark tells the programme the money could have transformed their lives: "We wouldn't have been forced into taking on loans from family and struggling. I could have a day or two days off, I could have had a weekend."
The Labour peer Lord McFall says that many homeowners are paying large amounts of extra interest: "In some of the case studies I've seen, people with a mortgage would be paying an extra £40-50,000 in the mortgage before it's completed. That is totally unacceptable."
At the time of the £13bn sale, the government bank, UKAR, told Lord McFall that homeowners should get better mortgage deals from Cerberus.
In a letter to the peer, UKAR said that "by returning ownership to the private sector the option to be offered new deals, extra lending and fixed rates should become available to them".
A UKAR spokesman told the programme that Cerberus had the ability to lend to the former Northern Rock customers and that UKAR believed they intended to do so.
"The reply to Lord McFall sent on behalf of the UKAR board of directors was based on information presented to UKAR and the board had no reason to disbelieve this at that time."
Panorama understands that the information presented to UKAR was in the pitch documents Cerberus submitted when it applied to buy the mortgages.
They said they planned to "evolve into an online challenger bank" providing customers with "a wider range of products".
Before the sale was finalised, however, the Financial Conduct Authority told Cerberus it couldn't offer new mortgages until it put the right systems and people in place. Nearly three years later, Cerberus still hasn't done that.
The private equity firm said: "Cerberus and our affiliates take great pride in being good corporate citizens who are committed to compliance with the strongest ethical standards and all legislative and regulatory requirements. We reject any allegations by BBC Panorama to the contrary."
Mistakes and complaints
Some of the homeowners say that as well as paying high interest rates, they have also been poorly treated by Cerberus.
Rachel and Adrian Neale, from Hinckley, fell into arrears on their mortgage when Rachel became seriously ill with Crohn's Disease.
They have paid off most of the arrears and have kept up their monthly payments with Cerberus's mortgage company, Landmark Mortgages. But Landmark keeps making mistakes and issuing default notices.
The couple say they have already made about 70 complaints and that nearly all have been upheld.
"You're in their palm basically," said Adrian. "They can do what they want, that's how it makes you feel, they can pull the rug from underneath you at any time."
It can take hours on the phone to sort out the mistakes and the default notices have badly affected Adrian's building business.
"I can't take on big jobs because I can't gain credit to fund a big job. Building merchants, credit accounts, I can't get access to it and it's held me back massively really in my business."
Landmark says it is committed to high standards of fair customer treatment: "Our staff are trained to deliver fair, consistent and right customer outcomes. Landmark promotes high conduct standards, adopting a tailored approach to customers experiencing financial hardship to ensure we find a suitable and fair solution."