LONDON—Britain’s six biggest power companies have been overcharging customers by about 5%, a U.K. government investigation said Tuesday, but the report blamed green-energy subsidies, a lack of competition and transparency and ineffective regulation for higher prices.

The Competition and Markets Authority said it found no evidence of wrongdoing by the companies. Instead, consumers have been ill-served by an energy market framework that resulted in prices that were too high by about 5%, or about $1.8 billion a year, the authority...