Up to a million homeowners may not see the full benefit if, as expected, the Bank of England reduces interest rates by a full percentage point tomorrow.
The Monetary Policy Committee meets today to discuss further rate changes and is coming under increasing pressure for a further significant reduction.
However, many people will not fully benefit from any cut as some lenders impose a collar on tracker mortgages, which sets a minimum level on the interest rate charged.
Nationwide and HBOS are among a host of lenders with collars, ranging from 1% to 4.5%, on some of their mortgages.
Ray Boulger, a senior technical manager at John Charcol, the independent mortgage experts, said: 'If the bank rate falls by more than a quarter of a point on Thursday, which seems virtually certain, about half of the borrowers with a collared tracker mortgage will definitely not see the full benefit, and some will not benefit at all,' he said.
He estimates this could be between 500,000 and a million borrowers.