BORROWERS COULD be in for a shock: their monthly mortgage payments will jump when they come to the end of cheap two-year fixes.

Back in November 2005, when the base rate was at 4.5%, you could fix your mortgage for two years at close to 4%. Portman, for example, was offering a two-year deal at 4.15%. Or there was a two-year fix from Britannia at 4.24%.

Borrowers rushed to snap up the cheap deals and cut their monthly payments. If you had a 25-year, £100,000 repayment mortgage at 4.15%, the monthly payments would be £536. You would pay £541 a month at 4.24%.

But times have changed. The base rate has gone up by 1.25 percentage points since August last year to 5.75%, and the very low rates are no longer available. James Cotton of L&C Mortgages, a broker, says: "Some borrowers will be in for a shock because they have been protected from the rate rises of the past year or so."

But experts still recommend that you look around for a better deal. If you stick with your existing lender you will probably end up paying the standard variable rate (SVR) when the fix expires. And that's expensive. For example, Portman, which is now owned by Nationwide, charges an SVR of 7.24%, so you would pay £722 a month on the £100,000 mortgage.

Some lenders charge a penalty if you switch to a rival lender, but most penalties expire at the same time as the fixed rate. Even if your loan carries so-called extended penalties that apply for several years longer than the fix, it can still be worthwhile.

A number of lenders have come out with best-buy fixes in the last week. Newcastle Building Society, for example, is offering a two-year fix at 5.45%, with a £599 fee. Or there is a five-year fix at 5.35% with a fee of £999. The monthly payments on the two-year deal would be £611, or £605 on the five-year fix.

Cheshire Building Society also has a two-year fix at 5.49%. The arrangement fee is £999, but there is a free valuation for people who are switching their mortgage.

The deals reflect the expectation in the City that the Bank of England will cut the base rate when it meets in December. In his latest report, Mervyn King, the governor of the Bank of England, signalled that two rate cuts were needed before summer to lift the economic gloom.

But if a fall in interest rates is on the cards, should borrowers opt for a fix? If they lock in now, surely they will miss out if rates come down?

Cotton says: "It all depends on your budget. If you choose a discount or tracker mortgage your loan will move up and down with the base rate. That's all very well if rates come down, but you have to be able to cope if rates go up."

At the moment, the best fixes are also cheaper than the best discounts. Nottingham Building Society has one of the top discounts on the market - a three-year deal at 5.69% with a £595 fee and free valuation and legal work for people who are switching their mortgage. It's more expensive than the low-cost fixes and the base rate would have to drop a long way before it became cheaper.