One energy company is facing investigation on a postcode lottery over bills, with another also facing action from the regulator.

The two, which have not been named, have been asked "serious questions" by Ofgem over price differentials.

One must explain why the cost of energy is apparently different in one area of the country compared with another where it is competing for business.

The pair will be named if an official investigation is launched, Ofgem said.

"We do have concerns over some premiums and have made it clear to the two suppliers concerned that we will take action if the differentials cannot be justified," said Sarah Harrison of Ofgem.