Calls for intervention in the energy market to cut bills have mounted after five of the Big Six suppliers increased standard variable tariffs

The government has given its clearest warning yet that it is preparing to cap energy prices, vowing to take “very decisive” action to address the “flagrant mistreatment” of millions of households on expensive tariffs.

Shares in both Centrica and SSE, the biggest household energy suppliers, fell by more than 1.5 per cent yesterday after Greg Clark, the business secretary, said that customers on standard tariffs were exploited, abused and milked. The remaining four members of the Big Six energy businesses are EDF Energy, Npower, Eon and Scottish Power.

Ministers were finalising “muscular and strong” proposals to intervene in the market, which they planned to publish very shortly, Mr Clark told MPs on the business select committee, indicating that they could do so before the election.