Borrowers risk paying hundreds of pounds in extra interest by signing up for mortgage deals which end sooner than expected.

There are a number of two-year fixed rates available, which actually end in less than 24 months. Bristol & West have a ‘two-year’ deal at 5.99 per cent but the fixed rate ends on 31 August 2009 – thish is often referred to as a short, or skinny, end-date. Given that it usually takes at least four weeks for a mortgage application to be processed, anyone signing up for that product now may only end up with the rate for 21 months. At the end of the fixed rate period, they will be charged Bristol & West’s standard variable rate, currently 7.84 per cent, unless they remortgaged in time. Someone borrowing £200,000 on an interest-only basis, would pay an additional £927 interest if they switched onto the SVR for the final three months.

Francis Ghiloni at Mform, a comparison site: “Many borrowers expect their rate to be fixed from the time the mortgage is completed, but most lenders offer deals fixed to a specific date. If you assume it takes at least a month to complete a mortgage – and it is usually a lot longer if you are moving house – then your mortgage may well end sooner than you are expecting. Borrowers therefore need to focus on the total cost of the mortgage over the term.”

If you are comparing two deals with similar rates and fees, the end date is very important – it may even work out cheaper to pay a slightly higher rate if the deal last for longer.

While short end dates can catch the unwary borrower out, deals that run until a specific date, as opposed to two or five years from completion, can be beneficial.

Ray Boulger at John Charcol, a broker, said: “The flip side is that if you sign up for a deal with a long end date you may get the preferential rate for longer than expected.”

For example, Yorkshire building society’s end dates are 28 February. It has a five-year fix at 5.79% which runs until the end of February 2013. So someone applying now, would have their rate secured for five years and three months, assuming it takes a month to complete.

Similarly, BM Solutions has a three year fix at 5.75%, which has an end date of 28 February 2011.

Nationwide is one of the few providers to fix from completion date, so anyone applying for one its two-year deals knows their rate is guaranteed for the full period.