Over a million small firms have been warned that their electricity and gas bills could increase by 100% this year, and are being advised to check their contracts to ensure they are not due to be automatically renewed.

Price comparison service Make It Cheaper advise that the increases will apply to any business on a fixed price energy contract which expires and is automatically renewed.

Small companies that signed energy contracts a year ago would have received a tariff of around 7p/kWh for electricity and 2.5p/kWh for gas.

Many utility companies renew a company's contract automatically without informing them of the increase in their charges - and both energy and gas prices have almost doubled over the last year.

"Businesses, particularly small businesses, are either totally oblivious to the renewal trap, or confused by the small print which can run to 9,000 words in tiny font," said Jonathan Elliott, managing director of Make It Cheaper.

"If you fail to serve the correct notice, you will fall victim to the trap and rule yourself out of the best deals available upon renewal."