According to comparethemarket.com, some 70 fixed price tariffs will come to an end this quarter, which would equate to a £200m “inertia windfall” for energy companies.

In April alone comparethemarket’s data showed that 30 fixed rate tariffs ended, creating an average annual increase of £359 per customer if they roll over onto the standard variable tariff.

While the majority of the tariffs ending are from smaller, independent providers, it is still estimated the increase will affect around 600,000 energy customers across the UK.

Peter Earl, head of energy at comparethemarket, said: “The cost of not actively monitoring your energy bills is clearer than ever, with energy companies potentially enjoying close to a £200m inertia windfall from customers who don’t choose to switch.

“A £333 increase in annual energy bills could be devastating to so many families across the UK. However, the good news is that this can be avoided – by switching back on to a competitively priced fixed tariff which best suits your needs.”

See YourMoney.com’s A guide to switching energy provider for all you need to know about moving from one supplier to another.