Which? has claimed that 95% of lenders have failed to pass on cuts in base rate to customers on a standard variable rate (SVR).
The consumer watchdog has also warned that a rise in the base rate of one percentage point would add over £50 to the monthly repayments of someone with a £100,000, 20-year mortgage.
It says its research shows that over one-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 were the only lenders who are part of the four biggest banking groups to pass on the full cut.
At 6.08%, Kent Reliance Building Society has the highest SVR on the market. The five other direct lenders with the highest SVRs are all building societies.
The average SVR is now 3.48% above the base rate, compared with 1.95% in September 2008.
Which? also claimed that seven in 10 people are worried about mortgage rates and two in 10 fear repossession.
Peter Vicary-Smith, Which? chief executive, said: “Millions of people are on variable rate mortgage deals and for many a rate hike could mean they’re facing real financial difficulties.