Ministers have set government departments guidelines on energy
The government has made "very poor progress" on reaching its own carbon emissions-cutting targets, MPs say.
Ministers want departments and agencies to reduce emissions by 12.5% by 2010/11 compared with 1999/2000 levels - and to be carbon-neutral by 2012.
But the environment audit committee said a cut of just 4% had been achieved by 2006/07.
Chairman Tim Yeo said this damaged the government's "moral authority" on environmental issues.
'Degree of confusion'
The committee said it was "extremely disappointed" that only 0.0004% of all electricity consumed by the government was generated on Crown property using renewable energy sources such as wind, solar or biomass power plants.
Ministers were relying "too heavily" on buying offsets to help achieve carbon neutrality, the report - Making Government Operations More Sustainable - said.
There was a "degree of confusion" about how the targets would be met and these "essential issues" should be worked out urgently, the committee added.
Mr Yeo, a Conservative former minister, said: "Until the government shows that it is living up to its commitments it will find it hard to maintain the moral authority to influence the rest of us."
The committee also expressed concern over the reliability of emission figures.
Members criticised the Ministry of Defence for claiming a big cut in emissions after it sold the defence agency QinetiQ.
In reality, the committee said, the government was simply moving these emissions "off-balance sheet" to the private sector.
A committee spokesman said: "The government has now stopped claiming this as a cut in emissions, but the committee warns it not to make similar claims in the future."
The Office of Government Commerce should annually publish details of the amount the Government expected to spend on offsetting emissions, it was added.