Ed Miliband has said there can be no justification for further gas and electricity price hikes until the "broken" energy market has been reformed.
The Labour leader said the announcement by regulator Ofgem that it was referring the sector for a full-scale competition inquiry was confirmation that he was right to argue the market was "rigged".
Speaking to the Federation of Small Businesses (FSB) in Manchester, Mr Miliband said a Labour government would legislate to give small firms the same protection as householders in the energy market.
"The Big Six energy companies have made huge profits at the expense not just of hard-working families but hard-working business people too. They have done it because the market just isn't working," he said.
"And while we reform that broken market, there can be no justification for further price rises. Because we now have official confirmation that these will be price rises taking advantage of a market that is rigged."
He said that Labour would establish a new energy regulator with powers to stop suppliers from rolling firms on to more expensive tariffs without their consent or hitting them with "crippling" back-bills going back years.
Chancellor George Osborne was also at the FSB attempting to woo small businesses with plans to make it easier for firms which had been turned down for loans to turn to other lenders.
He said the Government would consult on whether legislation was required to force lenders to release information on SMEs they reject for finance, so that they can be identified and approached by alternate credit providers.
"The success of small and medium sized business is key to the Government's long term economic plan," he said.
"That's why we are fully focused on making sure businesses can get the finance they need to grow and create jobs. This includes actively supporting innovative new forms of business lending.
"A big bank saying 'no' should not be the end of the line for a small business. Now, with our plan, it won't be."