A committee of MPs has accused ministers of "shifting the goalposts" to reduce the number of households in England classed as in fuel poverty.
The definition of fuel poverty would be changed by amendments to the Energy Bill so that 2.4 million were classed as fuel poor rather than 3.2 million.
The Environmental Audit Committee says that is unacceptable.
The government insists the changes help "to get a better understanding of the causes and depth of fuel poverty".
The cross-party committee's report said families were currently classed as fuel poor if they spent more than 10% of income on fuel "to maintain an adequate level of warmth".
Under the new definition, families would only be deemed to be in hardship if they had "above average fuel costs" leaving them with "a residual income below the official poverty line".
Committee chair Joan Walley said: "The government is shifting the goalposts on fuel poverty so that official statistics record far fewer households as fuel poor.
"The changes to the fuel poverty definition and target, in part being made through amendments to the Energy Bill, should be stopped unless the government is prepared to make a public commitment to end fuel poverty altogether."
On Saturday, Chancellor George Osborne outlined proposals to reduce energy bills by an average of £50 a year in an effort to offset rising costs.
He said the government would pay for some measures currently included in people's bills and the cost of insulating homes would be spread over a longer period.
But Ms Walley said the plan could end up making things worse for fuel-poor households.
"A short-term bid to cut bills must not throw energy and climate change policy off-course," she said.
"In the longer term, green levies could actually keep bills down if they drive energy efficiency improvements that cut the cost of heating our homes.
"Insulating homes and supporting green technologies is vital to help the fuel poor and cut the emissions causing climate change."
But a Department of Energy and Climate Change spokesman said: "The changes to the fuel poverty definition helps to get a better understanding of the causes and depth of fuel poverty, and to target policies more effectively.
"There is already a range of help for those in most need, including the warm home discount, winter fuel payments and cold weather payments."