Big Six energy suppliers

The Big Six were the United Kingdom's largest retail suppliers of gas and electricity, who dominated the market following liberalisation in the late 1990s. By 2002, six companies – British Gas, EDF Energy, E.ON, RWE npower, Scottish Power and SSE – had emerged from the 15 former incumbent monopoly suppliers (the 14 regional public electricity suppliers and British Gas).

In 2008, the Big Six still had over 99% of domestic and small business customers. They were vertically integrated in electricity, owning sufficient generation capacity to supply all their customers; while in gas, only Centrica (British Gas) owned production facilities. By the third quarter of 2019, after efforts by the regulator Ofgem to promote competition, their combined share in Great Britain was 70% for electricity supply and 69% for gas. The purchase of SSE's retail business in January 2020 by OVO Energy, a competitor founded in 2009, marked the end of the original Big Six.