British Gas Business will pay a penalty of £9.5m after its customers suffered billing and complaints issues, the energy regulator has said.
Inaccurate and delayed bills were given to business customers after the provider put in a new £40m computer system in 2014, Ofgem said.
The penalty will be paid as compensation and in a charity payment.
British Gas Business apologised and said good quality customer service had been restored.
Regulator Ofgem said that tens of thousands of customers were left with inaccurate and delayed bills after the new IT billing system was introduced.
More than 6,000 new business customers had difficulty in registering with the supplier. There were insufficient staff in place to deal with complaints, Ofgem added.
British Gas initially compensated some customers and itself reported the problems to the regulator.
It has since hired 350 extra staff to handle the backlog of complaints, and will now pay £9.5m in compensation to micro-business customers (which it will contact directly) and payments to the Money Advice Trust, which runs the Business Debtline.
Dermot Nolan, Ofgem chief executive, said: “Ofgem welcomes suppliers investing in new systems and recognises that this can be challenging. However, the £9.5m payment also sends a strong reminder to all energy companies that they must treat consumers fairly at all times, including while new systems are put in place.”
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.”
The penalty is smaller than some handed out by Ofgem, such as a £26m fine given to Npower in December 2015 for billing and complaints issues, and an £18m penalty given to Scottish Power in April following IT problems.