British Gas has been forced to pay £1.73m to customers affected by changes the energy supplier made to its prepayment plan, the watchdog said on Thursday.
The Centrica-owned supplier failed to inform some prepayment customers, many of whom are vulnerable, about a change of top-up provider and minimum top-up spend.
Some may have had their supply of gas and electricity cut off as a result, Ofgem said, or suffered "wasted journeys to shops where they could no longer top-up".
In addition, customers were unable to contact the British Gas enquiry line on the day of the change.
To compensate affected customers, Ofgem has ordered British Gas to pay £1.48m for its failings and an additional £250,000 into the energy redress fund.
When the new top-up provider took over on January 1, British Gas failed to notify some 270,000 prepayment customers about the change from Paypoint to Payzone.
"This meant that some customers, many of whom are in a vulnerable situation, may have suffered detriment through wasted journeys to try and top-up at shops which no longer offered the service, or in some cases may have gone off supply," Ofgem said.
British Gas only informed these customers about the changes in late January and failed to include contact details should customers have further questions.
Additionally, British Gas chose not to operate its general enquiries line on January 1, Ofgem said.
Due to the steps British Gas has taken, the regulator has decided not to take formal enforcement action.
The company said: "We recognise that this transition was not as smooth as it should have been and we would like to apologise again to any customer impacted."
Citizens Advice boss Dame Gillian Guy said: "It is unacceptable that poor communications put people at risk of being cut off from their energy supply in the middle of winter.
"Households with pre-payment meters are disproportionately likely to be on lower incomes, have children or include people with health conditions. So it's critical that suppliers take extra care and make sure changes like this don't put people at risk."