
Energy companies will be banned from charging "unjustified" prices under new rules proposed by the regulator Ofgem.
Prices, including those for pre-payment meters, will have to be "objectively" justifiable under new conditions.
Ofgem started the investigation in February 2008 after all the leading suppliers imposed big price rises.
It has been looking for evidence of collusion among suppliers and to see if the wholesale markets in which gas and electricity are traded are working.
Ofgem had previously said that there was no evidence of collusion between the "big six" suppliers in setting prices.