More than a quarter of UK homeowners on a fixed-rate mortgage are heading for sharp increase in monthly payments before the next election, in a financial timebomb that will rock the Conservatives just as voters prepare to choose the next government.

With the Bank of England expected to increase its key interest rate next week for the 13th time, figures shared with the Guardian by UK Finance, the banking industry trade body, show more than 2.4m fixed-rate homeowner deals will expire between now and the end of 2024.

The prospect of millions of households facing a dramatic rise in borrowing costs comes after a week of renewed turbulence in financial markets as City traders bet the Bank rate would reach close to 6% before Christmas.