British Gas and SSE, which have over 40% of the market between them, now use more coal to produce electricity than they did in 2005, The Independent has reported.
Experts said their reliance on coal – the dirtiest form of fossil fuel, which produces twice as much CO2 as gas – was undermining attempts to cut the UK’s carbon emissions through renewable supplies.
During the past 10 years the percentage of electricity generated from renewable sources has grown by 400%, yet total carbon emissions from generation have only fallen by around 8%.
This is because while the Big Six energy companies are now buying more than a third of the energy that they sell from polluting coal-fired power stations, they have cut back on buying power from more expensive but greener gas-fired power stations.
Environmental campaigners said this undermined the companies’ claims to be “going green” and called on the government to step in and take action.
The statistics come as the campaign group 38 Degrees and the Big Deal, a consumer collective, prepare to launch a drive to persuade consumers to switch energy suppliers to companies offering zero-carbon electricity.
The Clean Energy Switch will attempt to use the bargaining power of tens of thousands of people to cut the cost of buying renewable energy to below levels comparable with what most people pay for conventionally generated electricity.
They hope it will put pressure on the Big Six to cut their reliance on fossil fuel generation and spur further investment in renewable technologies.
The plan is backed by Labour, the Liberal Democrats and the SNP at Westminster.
Last year, 22% of the electricity sold by British Gas came from coal generation. In 2005 the figure was 14%.
Just over 31% of SSE’s electricity was generated from coal last year, compared with 29% in 2005.
Other companies, however, appear to have been more successful at reducing their coal dependency.
EDF produced 46% of its electricity from coal in 2005, but this has been cut to 26.8% by 2014.
Electricity generated by coal emits around 910 grams of CO2 per kilowatt hour compared with 390g for gas generation, and nothing for nuclear or renewable power.
Experts said many companies had chosen to continue buying and using electricity generated from coal because it was more profitable to do so.