Consumer group Which? says that 95% of lenders have not passed on cuts from the historically low base rate of interest.
And lenders are being warned that with many people trapped on their SVRs as they are no longer eligible to meet criteria for the best deals, a rate increase could leave thousands of households in financial peril.
Millions of people have reverted to their lender's SVR since the base rate was cut to 0.5% in March 2009, but some could be paying rates up to 12 times that amount.
Cheltenham & Gloucester and Lloyds TSB Scotland were the only lenders which are part of the four biggest banking groups to pass on the full cut.
At 6.08%, KRBS has the highest SVR on the market - more than 12 times the base rate.
The five other direct lenders with the highest SVRs are all building societies (Newcastle BS, Nottingham BS, Shepshed BS – all 5.99%; Darlington BS, Marsden BS – 5.95%).
The average SVR is now 3.48% above the base rate, compared with 1.95% in September 2008.
And there is clear worry about the effects of any increase passed on by a base rate rise, with seven in ten people worried about mortgage rates and two in ten fearing repossession
Just a 1% increase to the base rate would add over £50 to the monthly repayments of someone with a £100,000, 20-year mortgage.
With evidence showing that low interest rates have helped keep thousands of homeowners' heads above water, such a rise could tip many into lasting problems with their mortgages.
"Millions of people are on variable rate mortgage deals and for many a rate hike could mean they're facing real financial difficulties," Which? chief executive Peter Vicary-Smith, said.
"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."
The Council of Mortgage Lenders has hit back at the claims, saying that the base rate does not directly govern mortgage rates, and that the requirements for lenders have changed dramatically since the onset of the economic crisis
"Lending rates are fundamentally driven by the cost of funds, not the base rate, although the two were more closely correlated before 2008," CML director general Michael Coogan said.
"But this apparent historical relationship has been blown apart by the move to an unprecedented low base rate since March 2009.
"Since the onset of the financial crisis, firms have been operating in lending and funding markets that have changed dramatically, and we have been reinforcing the message that base rate is not a proxy for the funding costs for lenders."
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