Shares in Centrica and Scottish & Southern Energy were down on the FTSE 100 after energy regulator Ofgem said it would be introducing a new regime to prevent energy companies overcharging its customers.
Ofgem said that energy companies would be required to include more detail on customer's energy bills, including the amount of energy used, the cost of the energy, the annual price and if customers could get a better deal on another tariff.
Ofgem conceded that such bills could drive up processing costs but added that this cost could be covered by customers.
More worryingly Ofgem said that it was seeking answers from two of the big six energy company's about overpricing, however the regulator did not say which companies where under investigation.
By 10:06 shares in Centrica (owner of British Gas) were down 0.61 per cent to 294.90 pence per share and shares in Scottish & Southern Energy fell 0.27 per cent to 1,116.00 pence per share.
Overall the FTSE 100 dropped 1.28 per cent to 4,854.00.