Licensees are being urged to be wary of 'win back' calls from British Gas which could leave them tied in to two contracts and facing penalty fees.
Watchdog Energywatch claims that small businesses that have switched from British Gas Business and agreed terms with a new supplier are getting last-minute sales calls from the company offering to beat the competition's new deal.
The watchdog said it has received several complaints from businesses claiming to have been charged penalties by suppliers after accepting a new deal with British Gas.
Paul Savage, business services manager for energywatch, said: "Where a business has agreed to move suppliers, even over the phone, the new supplier is entitled to enforce its new contract, even before it has actually taken over the supply.
"As a result, small businesses can easily find themselves tied into two contracts with different suppliers at the same time and at risk of paying substantial penalties."
John Holbrow, chairman of the Federation of Small Businesses, said: "British Gas knows full well that small businesses can't get out of these contracts and we would ask them to stop their hard-sell techniques."
A spokesman for British Gas Business said: "Ofgem (the electricity and gas regulator) has looked at this already and has concluded we are not breaking the law.
"What we do is offer customers choice. They can take or reject the offer. We are asking Ofgem to end early-termination fees for customers who change their minds."