Research by Which? Money shows 95% of lenders failed to fully pass on cuts in the base rate to their SVR mortgage customers.
It also reveals that more than a fifth of lenders have increased their SVR since the base rate hit an all time low of 0.5% in March 2009.
Cheltenham & Gloucester and Lloyds TSB Scotland are the only lenders that are part of the four biggest banking groups to have passed on the full cut.
The average SVR is now 3.48% above the base rate, compared with 1.95% in September 2008.
And Which? says that with many borrowers trapped on SVR mortgages because they are unable to remortgage, any further increases in SVRs could have serious implications.
Peter Vicary-Smith, chief executive at Which?, says: “Millions of people are on variable rate mortgage deals and for many a rate hike could mean they’re facing real financial difficulties.
“Banks have enjoyed increased margins on mortgages for the last few years and when the base rate rises again, few lenders will be able to justify passing the full amount onto their SVR customers.”