Thousands of borrowers will miss out on cheaper mortgage bills because some of the biggest lenders prohibit interest rates on their tracker loans from falling below a minimum threshold.

Economists say interest rates could fall in the coming months as the Bank of England moves to boost the economy. The Bank cut rates by 0.5 per cent to 4.5 per cent last week, and experts say rates could fall to 2 per cent next year. But thousands of borrowers on tracker mortgage deals will not benefit from lower monthly bills because their lender will not pass on the cuts.

Halifax Bank of Scotland (HBOS), the country’s biggest mortgage lender, which is set to be taken over by Lloyds TSB, will not cut tracker rates if the bank rate falls below 3 per cent. Nationwide Building Society, the second-biggest lender, will also refuse to pass on any decreases if the base rate sinks lower than 2.75 per cent.

Abbey has a 3 per cent cut-off point on some older trackers, while building societies including Yorkshire, Skipton, Norwich and Peterborough and Scarborough operate similar thresholds on all their tracker deals.

A homeowner with a £150,000 HBOS home loan pegged 1 percentage point above the base rate would miss out on savings of £240 a year if the base rate was cut to 2.75 per cent as his mortgage rate would stay at 4 per cent, rather than 3.75 per cent. If the base rate was cut to 2 per cent, he would miss out on savings of nearly £1,000 a year.

HSBC does not have a lower threshold on its deals, but includes a caveat in its terms and conditions allowing it to not pass on rate cuts if there is a “significant” change in the mortgage market. However, a spokesman said that it was “committed to upholding the agreement with our customers”.

In an additional blow for HBOS customers, the lender this week told borrowers it was changing its promise that its standard variable rate (SVR) would not be any higher than 2 per cent above base rate. From next month the lender is increasing this margin to 3 per cent.

Tracker-rate loans have become increasingly popular in recent years as borrowers opted for loans guaranteed to move in line with the base rate. A quarter of all home loans taken out last year were tracker deals, says the Council of Mortgage Lenders.

Many homeowners are likely to be surprised to learn that their tracker deal is not quite what it seems, as many lenders did not highlight this aspect of the small print to customers or mortgage brokers. Brokers said lenders should contact brokers and customers to remind them of the limits.

Chris Cummings, director-general of the Association of Mortgage Intermediaries, said lenders should contact borrowers and brokers to remind them of the clauses. “Brokers are beholden to lenders to explain such hidden catches so they can explain them to clients. By and large, lenders have not done this. In the current climate, they should reissue this information.”

Ray Boulger, of John Charcol, the mortgage broker, said: “If people buy a tracker mortgage they expect their rate to move in line with base rate. They will be shocked to learn that their rates won’t do that. A tracker ought to do what it says it on the tin.”

Yorkshire Building Society said that it had put a floor on its tracker rates in order to make sure it could still offer competitive savings rates, that are necessary to keep attracting new funding.

Matthew Carter, Nationwide’s director of mortgages, said: “We have to balance mortgages and savings, and there will be some levels beyond which you can’t cut the rates and still call them savings accounts.”

Halifax defended changing the terms of its SVR, saying that its current rate of 6.5 per cent was one of the most competitive. A spokesman said: “There is no legal or regulatory requirement to impose a maximum limit on the SVR, we actually do so to provide some certainty for customers.”