Consumer campaign group Which? has accused banks and building societies of putting the squeeze on homeowners with a standard variable rate mortgage.
Which? suggests that thousands will be pushed into financial difficulty when interest rates rise.
An increasing number of homeowners - over 40% - are on standard variable rate mortgages and they're paying higher interest.
After the Bank of England slashed its base rate in the financial crisis, 95% of lenders failed to pass on the cuts to their standard variable rates. Since then, more than 20% have actually raised them. With the highest at 6% interest - double the best value mortgages - lenders admit that they have been rebuilding profits.
In an interview with the BBC Michael Coogan from the Council of Mortgage Lenders said: "I think what we have is the banks and building societies looking to try and restabilise the system that was in shock in 2008, recapitalise their organisations, deal with past losses, deal with the risk of potential future losses and at the same time keep their customers as happy as possible through an economic cycle."
It's hard to keep customers happy though because alternative loans are currently so difficult to get hold of. The danger comes for many homeowners who are on high standard variable rates but they're financially stretched so other lenders aren't interested in them, they're trapped and they would really suffer if interest rates went higher.
David Hollingworth, London & Country Mortgages: "I think it's highly unlikely to say the least that lenders won't look to push their standard variable rate up when base rate moves. In fact, actually quite the reverse, they'll look to push it up by more than any base rate increase and that's where vulnerable borrowers really stand to lose."
The Fellowes' bank and other lenders say they have to pay more for funds than the Bank of England's base rate, but Mark and Gwyneth have managed to switch to another mortgage and they're saving £120 a month