The US Environmental Protection Agency (EPA) is guilty of consistently distorting science for political reasons and pressuring its staff scientists to water down the green regulations and guidelines they are working on.

That is the conclusion of a major investigation from the Union of Concerned Scientists (UCS), which found that of nearly 1,600 staff scientists over half claim to have experienced political interference in their work in the past five years.

Furthermore, almost a third of respondents claimed to have personally experienced frequent or occasional "statements by EPA officials that misrepresent scientists' findings", while 31 per cent felt they could not speak candidly within the agency.

Francesca Grifo, director of UCS's Scientific Integrity Program, said the investigation revealed "an agency in crisis" and warned that "distorting science to accommodate a narrow political agenda" posed a serious threat to the environment.

The report also found that political interference is most pronounced in offices where scientists are involved in writing regulations and at the National Center for Environmental Assessment, where many of the risk assessments that shape future legislation are undertaken.

"The investigation shows researchers are generally continuing to do their work," said Dr Grifo. "But their scientific findings are tossed aside when it is time to write regulations."

Moreover, much of the interference was alleged to have come direct from the White House's Office of Management and Budget (OMB) with one respondent claiming that "OMB's power needs to be checked as time after time they weaken rulemakings and policy decisions to favour industry."

The report will increase pressure on the EPA as the agency prepares to face a series of rulings over its failure to impose stringent enough environmental regulations.

Congress is investigating administration interference in a new chemical toxicity review process and the House Oversight and Government Reform Committee is expected to hold a hearing into political interference in the new EPA ground-level ozone pollution standard early next month.

Meanwhile, the EPA is also preparing to defend itself against legal action from California and 12 other states over its refusal to grant approval to the states' plans to impose their own car emissions standards.