Centrica, which operates British Gas, suggested it would not be able to continue to supply household energy if Ed Miliband's plan to freeze prices came into force.
"If prices were to be controlled against a background of rising costs it would simply not be economically viable for Centrica, or indeed any other energy supplier, to continue to operate and far less to meet the sizeable investment challenge that the industry is facing," a statement said.
"The impact of such a policy would be damaging for the country’s long term prosperity and for our customers.”
British Gas' residential business increased profits by 3.2% to £356 million in the first half of this year, Centrica announced in July.
As Ed Miliband clashes with the energy giants on prices, it is worth knowing how your expensive energy bills are actually made up.
A leading competition lawyer says an energy price freeze could leave Labour open to challenges from the courts or the European Commission.
Whether or not you approve of a state intervention into industry, there are at least two big potential obstacles for an energy price freeze.