The government has admitted that it is powerless to stop energy firms passing on the cost on its fuel assistance package onto customers.
Business Secretary John Hutton said he could not stop firms increasing bills in response but said there would be "no justification" for them doing so.
The plan includes half-price insulation for all households and a freeze on this year's bills for the poorest families.
The measures have been criticised as "flimsy" and "disappointing".
Gordon Brown said he hoped for a "sea change in energy efficiency" and help for the poorest households this winter.
But anti-poverty campaigners say while the £910m package is a step in the right direction, it does not go far enough.
Mr Brown announced the package of measures, drawn up after weeks of negotiations with energy companies, to help people cope with rising energy bills.
The package is being financed by the companies themselves.
But Mr Hutton said the government could not fix prices to ensure customers did not end up footing part of the bill themselves.
But David Porter, chief executive of the Association of Electricity Producers, told the BBC earlier "It remains to be seen just how much of it ends up on the customers' bill in the longer run.
"Whenever people impose costs on an industry like ours the bill, to some extent, always ends up with the customer."
Labour MP Frank Field, who led a successful backbench revolt over the abolition of the 10p tax band, said after a summer spent "roaring about the package" the government had actually produced "a mouse of a proposal".
Another Labour MP, Alan Simpson, chairman of the Parliamentary Warm Homes group, told the BBC that an extra £74m pledged towards the Warm Front insulation scheme simply reversed existing cuts to its budget.
He said Britain should look to other European countries for a more radical approach - such as allowing price increases to be set by an independent regular.
"We have ducked this notion of intervening, and have gone to the industry sort of cap in hand," he said.
Gas and electricity watchdog Energywatch said the response to growing fuel poverty was "too little, too late".
Chief executive Allan Asher added: "The lack of political will to tackle fuel poverty is not just disappointing, it approaches negligence."
Help the Aged described the package as "flimsy and failing" which would do little to help the elderly while Tony Woodley, joint leader of the Unite union, said bigger steps were needed and promised a windfall tax a central theme of this month's Labour conference.
Lib Dem leader Nick Clegg said the government should have been tougher with the energy companies, adding: "These measures will not do nearly enough to help the millions of people who will struggle to heat their homes this winter".
And for the Conservatives, shadow business secretary Alan Duncan, added: "Despite all their grand promises of cash payouts, all Gordon Brown has been able to offer is to restore a budget which they cut last winter and a package of measures that should have been implemented years ago."