Chief executive Sean Gardner said research showed that 450,720 fixed-rate mortgages were completed in 2003, which amounted to 36 per cent of the total loans taken out for house purchase.
The average rate for a fixed-rate deal in 2003 was 4.23 per cent.
But the Bank of England has increased interest rates since then and the threat of further rises has pushed up fixed-rate deals ,with the average now 5.5 per cent.
Any homeowner failing to find a new deal and moving on to a standard variable rate faces an even bigger rise – the average is 6.64 per cent.
Decisive
On a £100,000 mortgage, such a rate will cost £692.05 a month in repayments compared with £546.47 for the average fixed-rate of 4.23 per cent in 2003 , an increase of £145.58 a month or £1,746.96 a year.
On an interest-only basis, the monthly repayments for a 6.64 per cent mortgage will be £553.33 compared with £352.50, an increase of £200.83 a month or £2,409.96 a year.
Mr Gardner said: "Homeowners who took out fixed rates two years ago received fabulous deals and have done very well.
"However, the interest rate climate has changed decisively since then and they have to take action or risk a massive rise in their monthly mortgage payments. Doing nothing is not an option.
"Someone moving from a two-year fixed rate they took out in 2003 to the standard variable rate would really suffer.
"A £200-a-month increase on the average £100,000 mortgage would raise their monthly payments by more than 50 per cent. It is essential they shop around for the best deal."