Gordon Brown has been warned not to "rat" on EU pledges to generate 20% of energy from renewable sources by 2020.
The aim is for 20% of EU energy to come from renewables by 2020
Acting Lib Dem leader Vince Cable said the PM could end up looking "less green than Blair", after a minister said that Britain's share should be 10%.
Mr Brown said he was "absolutely committed" to the 20% pledge but said individual targets were not yet set.
Mr Cable said Mr Brown, for the sake of the environment and his own reputation, must not "rat on the renewable power".
The exchange at prime minster's questions followed reports that ministers were planning a U-turn on pledges to ensure that 20% of European energy comes from renewable sources, like wind and solar power, by 2020.
It was reported that leaked documents showed the commitment was expensive and faced "severe practical difficulties".
Mr Cable asked Mr Brown: "If the government is fully committed to the 20% target for Britain, why did his own energy minister go on television yesterday and say he wanted it cut from 20% to 10%, under pressure from the nuclear lobby?
"Does the prime minister not realise that if he rats on renewable power, not only will it damage the environment, he will also drag his own environmental reputation to that of his friend George Bush?"
Mr Brown said the government was committed to the overall 20% pledge - but proposals for individual countries to meet had not yet been published. Once it had been, he said there would be a consultation on that.
But he said it could lead to "difficult decisions" for the opposition parties as they would include plans for a Severn barrage, and extensions of offshore and onshore wind turbines.
"I believe the Conservative Party has been totally opposed to something that is necessary to meet our renewable targets," he added.