The Bush administration has been continually accused of systemic tampering with the work of government climate scientists to eliminate politically inconvenient material about global warming.

At a hearing of Congress, scientists and advocacy groups described a campaign by the White House to remove references to global warming from scientific reports and limit public mention of the topic to avoid pressure on an administration opposed to mandatory controls on greenhouse gas emissions.

Such pressure extended even to the use of the words "global warming" or "climate change", said a report released yesterday by the Union of Concerned Scientists and the Government Accountability Project. The report said nearly half of climate scientists at government agencies had been advised against using those terms.

Yesterday's hearings, overseen by the new Democratic chair of the House committee on oversight and government reform, Congressman Henry Waxman, follow years of complaints by scientists that the Bush administration was seeking to put its own spin on scientific research at government agencies. They also complain of a reduction in funding for climate research since the 1990s.

The committee was warned that the campaign by the Bush administration discouraged free academic inquiry. "If you know what you are writing has to go through a White House clearance before it is to be published, people start writing for the class," said Rick Piltz, a former senior associate at the US Climate Change Science Programme. "An anticipatory kind of self-censorship sets in."

The balance appears to have shifted somewhat since the Democrats took control of Congress this month. At least five bills proposing mandatory caps on greenhouse gas emissions - an idea that is anathema to the White House - have been introduced in the House and Senate.

However, Mr Piltz told Congress even he was taken aback by the extent of the political interference, in technical reports, public meetings as well as exchanges with the media, in which scientists were assigned minders from the administration.

In the survey of 1,600 government scientists by the Union of Concerned Scientists, 46% had been warned against using terms like global warming in speech or in their reports. The scientists interviewed were working at seven government agencies, from Nasa to the Environmental Protection Agency.

Forty-three percent of respondents said their published work had been revised in ways that altered the meaning of scientific findings. Some 38% said they had direct knowledge of cases where scientific information on climate was stripped from websites and printed reports.

"There were a very large number of edits that came at the 12th hour after all the earlier science people had signed off," said Mr Piltz, who eventually resigned from his job because of such pressure. In one such case, a White House appointee, Phil Cooney, demanded 400 last-minute changes which significantly changed the meaning and tone of the report.

No detail was beyond the scrutiny of administration officials, it seemed. Drew Shindell, a scientist at Nasa's Goddard Institute for Space Studies, described how officials repeatedly objected to the title of a report which measured rapid warming in Antarctica before dictating their own choice. "Word came back from above that it should be: 'Scientists study Antarctic Climate Change'," Dr Shindell said. "I thought it was so watered down it would be of little interest to anybody."

Much of the testimony yesterday centred on the influence exerted by Mr Cooney, a former lobbyist for the petroleum industry who was put in charge of the Council on Environmental Quality. Mr Cooney now works for Exxon Mobil, the committee was told. In one instance, Mr Cooney personally edited out a key section of an Environmental Protection Agency report to Congress on the dangers of climate change. "He called it speculative musing," Mr Piltz said.

Mr Waxman said he knew of further evidence of such tampering but had been stonewalled by a White House which had repeatedly resisted requests for documents about Mr Cooney's involvement in controlling information.