British Gas Business will pay a penalty of £9.5 million due to customers suffering billing and complaints issues.
Ofgem said bills that were wrong and delayed were given to business customers after the company fitted a new £40m computer system.
The penalty will be paid as compensation as well as a charity payment.
British Gas Business has apologised and said good customer care service has since been restored.
A huge amount of business customers faced difficulty when signing up with the supplier, more than 6,000 were affected by the issue.
There also wasn’t enough staff in place to take care of complaints coming in.
Gab Barbaro, managing director of British Gas Business, said: “We invested in a new billing system so we could improve the service we provide to our business customers.”
“At the time, this was a major undertaking – merging nearly 100 different systems into one. It didn’t go as smoothly as we would have liked so we reported this to Ofgem as a priority.”
“We have restored a very good quality of customer service, and more and more of our customers are going online to use the new billing system. I’d like to apologise to our business customers for any inconvenience caused when we were implementing the system.”