The six largest suppliers' market share was 79 percent for electricity and 78 percent for gas in December 2017, down from 84 percent for both gas and electricity a year earlier, an Ofgem report said.

The report comes as Britain's energy firms are under pressure to reduce bills and as Prime Minister Theresa May this week introduced new legislation to force Ofgem to cap what she called "rip-off" prices until 2020.

Last year, 5.1 million electricity consumers and 4.1 million gas consumers in total switched supplier, the highest number for almost a decade.

More than a third of these customers switched away from the big six to a challenger firm, Ofgem said.

Britain's big six energy suppliers are Centrica British Gas, SSE (>> Scottish and Southern Energy), E.ON, EDF Energy (>> Electricité de France), Innogy's (>> innogy SE) Npower and Iberdrola Scottish Power.

(Reporting By Susanna Twidale, editing by David Evans)