Borrowers whose fixed-rate mortgage deals are due to finish in December have been warned their monthly repayments could increase by up to a third.

Marketing information group CACI said approximately 287,000 households will see their current fixed-rate deal expire between now and the end of the year.

The group said interest rates on fixed-rate home loans reached their lowest point in December 2005, with rates averaging 4.7%.

And with many experts predicting a further 0.25% hike in interest rates between now and the end of the year, the best fixed-rate loans are likely to be about 6.25%, a 33% rise for people who previously fixed at 4.7%.

Going from a rate of 4.7% to one of 6.25% on a £100,000 mortgage will add nearly £100 a month to repayments.

The group believes another 320,000 households are likely to come to the end of fixed-rate deals between January and August next year, and they could face similar hikes.

CACI said it believes wealthier homes are likely to be the hardest hit, as they are more likely to have taken out a fixed-rate loan with a rate of less than 5% than poorer households.

CACI director Penny Dryden said: "Home ownership is such a big priority in the UK that people have stretched themselves financially to get on the housing ladder, or to upgrade to a larger home or more desirable area.

"A small but significant proportion of the market will be affected by households coming to the end of their fixed-rate term."

The research also found that homeowners are increasingly turning to fixed-rate mortgages in the face of rising interest rates.

It said 72% of all mortgages taken out between January and April this year were fixed-rate loans, compared with 50% during the same period two years ago.

And lenders are reacting to the increased demand for a variety of fixed-rate products. The Halifax has launched a 25-year fixed-rate mortgage, joining four other lenders who now offer homeowners the chance to take out fixed loans for 25 years or more.

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