Delays in getting help to many vulnerable people meant they were unable to benefit from extra insulation which could have substantially cut their heating costs during the severe winter weather of 2012-13.
British Gas apologised for its failure, and said the bad weather of this period was a major reason why it had not met its targets.
But it added that, ultimately, it helped vulnerable customers achieve savings of £110million on their energy bills.
Senior Ofgem partner Sarah Harrison, however, called British Gas’s failure to meet its environmental obligations “unacceptable”, and added: “Thousands of households had to wait for energy efficiency measures, like insulation, to be installed during the winter.”
Work on installing the energy-saving measures should have been completed by the end of 2012.
Several other energy firms are still waiting to learn how much Ofgem will ask them to pay for their failure to meet the same targets, including Scottish Power and Scottish and Southern Energy (SSE).