The Bush administration is refusing to turn over documents involving the president's role in decisions about environmental policy.

The White House asserted executive privilege in a letter to a U.S. House of Representatives government oversight committee Friday, just before lawmakers were to vote on holding two high-level administration officials in contempt of Congress.

The committee chairman put off the vote while deciding what to do next.

The officials facing the contempt vote, including Environmental Protection Agency (EPA) Administrator Stephen Johnson, refused to turn over documents involving Johnson's decision last December to deny permission for states to enact their own vehicle emission standards.

The decision came hours after President George Bush signed a sweeping new energy bill that increases fuel efficiency standards by 2020. 

Many U.S. presidents have claimed executive privilege, although it is rarely used. 

It asserts that presidential deliberations with advisers are confidential and protected under the concept of separation of powers in the constitution.

President Bush previously cited executive privilege in refusing to allow his chief of staff, Joshua Bolton, and former counsel Harriet Miers to testify before Congress on the firings of several federal prosecutors.