According to MoneySuperMarket.com 13 fixed rate energy tariffs are due to expire at the end of August, including several from Scottish Power, Npower and Sainsbury’s Energy. Customers on these tariffs that roll over automatically to a standard tariff could end up paying anywhere from £15.09 to £148.07 more per year for their energy.

Clare Francis, editor-in-chief at MoneySuperMarket.com, said: “Fixed energy deals have been a popular choice for bill payers looking to protect themselves from price hikes. With so many coming to an end over the next few months, it’s crucial those affected move to a new tariff to avoid being caught out by rocketing bills.”

The current leading tariff – First Utility’s iSave Fixed September 2015 tariff – has an average bill of £992.45, £212 cheaper than Scottish Power’s standard tariff.

However consumers looking to switch energy providers should time their move carefully, as exit fees can be as much as £70 and the transition can take between four and six weeks.

Francis continued: “Failing to act when an existing deal ends means bill payers will pay over the odds for the gas and electricity they use. Switching energy providers is really simple – you can find the cheapest deal and apply in less than 10 minutes, so there really is no excuse and it can result in big savings.”