A top official at the US Environmental Protection Agency confirmed that the agency denied strong carbon emissions limits proposed by California after the White House intervened, it emerged today.

But the official, who resigned from the agency earlier this month, told congressional investigators that he was instructed not to reveal whether George Bush or other White House officials played a personal role in the controversial blockage of California's pollution rules.

The EPA associate deputy administrator, 31-year-old Jason Burnett, told the oversight committee of the House of Representatives that agency chief Stephen Johnson was prepared to approve a waiver allowing California to set strong limits on greenhouse gas emissions.

Trained staff members at the EPA had unanimously advised Johnson that US clean air laws supported California's plan and that the Bush administration should support it.

Burnett agreed, according to an interview summary the oversight panel released today. He told Congress that "all EPA recommendations that I am aware of ... were either supporting granting the full waiver or granting partial waiver".

However, Johnson shifted from support for at least a partial waiver in the autumn of last year to a denial of California's request in December. The state would have required a 30% reduction in tailpipe emissions from cars by 2016, dealing a blow to auto industry profits.

When asked whether Johnson spoke with the White House before his position changed, Burnett said: "I believe the answer is yes." Bush aides also had "input into the rationale" for the December denial, according to Burnett.

California has sued the Bush administration to force an acceptance of its environmental standards, with 16 other states waiting in the wings to join in the new emissions limits.

During the investigation into EPA's denial - which marked the first time in 40 years that the agency blocked a rule approved by the state - congressional aides examined 27,000 pages of documents and took testimony from eight agency officials.

"Clearly the Bush administration at the highest levels killed the California waiver," one of the state's Democratic senators, Barbara Boxer, said in a statement.

"The people of California and the other states have a right to know how this indefensible decision happened, and we demand that EPA and the Bush White House turn over the documents we have asked for immediately."

An EPA spokesman shrugged off today's congressional report as a retread.

"Distraction-oriented, political tactics of the committee will not keep EPA from moving forward, tackling tough issues and putting into place the most health-protective standards ever," the spokesman told the Associated Press.

© 2008 The Guardian