Ofgem said British Gas had paid £10m into a trust for vulnerable customers to settle a fierce row over pricing.
The donation is almost level with the record £10.5m fine Scottish & Southern Energy paid in April for “prolonged and extensive” mis-selling on the doorsteps of homes across the country.
British Gas was expected to announce its “donation” at the end of last week but it only came to light when sources contacted The Daily Telegraph on Monday morning.
The regulator claimed that between 2006 and 2011, British Gas failed to “round down” the calorific value, a charge that covers the heat or thermal energy in the gas a customer uses.
British Gas, the UK’s biggest supplier, used four decimal points, when Ofgem asks companies to use just one.