OTTAWA -- A large survey of science professionals in the federal public service has found that almost 25 per cent of respondents say they have been directly asked to exclude or alter information for "non-scientific reasons."

Some 71 per cent of those surveyed said political interference is compromising policy development based on scientific evidence, and almost half of those who took part said they were aware of cases in which their department or agency suppressed information.

The study, entitled "The Big Chill," was commissioned by the Professional Institute of the Public Service of Canada, and paints a disturbing picture of government scientists who feel they are being muzzled.

More than 4,000 federal scientists -- out of more than 15,000 who were invited --responded to the union-commissioned, online survey handled by the polling firm Environics.

"A chill has settled on federal government science that is even greater than that suggested by the cases so far reported by the media," Gary Corbett, the president of PIPSC, said Monday.

Federal Information Commissioner Suzanne Legault is already conducting a study of how communications policy changes under the Harper government have clamped down on the sharing of government science with the public.

Legault was spurred to investigate the issue by a lengthy report from the Environmental Law Centre at the University of Victoria and the ethics advocacy group Democracy Watch, which included a score of anecdotes from six different government departments or agencies.

The PIPSC survey, which was conducted June 5-19 and surveyed 4,069 of the union's 15,398 members, adds statistical heft to that anecdotal evidence.

The responses came from across more than 40 government departments and agencies and included 670 Environment Canada scientists, 651 from Health Canada, 427 Defence department employees, 343 from Fisheries and Oceans, 335 from the Canadian Food Inspection Agency and almost 300 each from Agriculture Canada and Natural Resources Canada.

Greg Rickford, the Conservative minister of state for science and technology, said in an email that the Conservatives have made "record investments in science."

"Science can power commerce, create jobs, and improve the quality of life for all Canadians," said the junior minister's email.

Through his office, however, Rickford did not respond to questions about the issue at hand: the alleged muzzling of scientists and the suppression of science in policy development.

A government official, speaking on background, said Environment Canada scientists alone attended 300 conferences in 2011, published 600 articles and participated in some 1,200 interviews.

The exchange with Rickford's office may help to illustrate the vast chasm between the perspective of elected officials and public servants.

The Conservative government, it appears, believes communication needs are easily met with carefully scripted and vetted talking points, even if off topic. Federal scientists, on the other hand, may feel differently.

Fully 90 per cent of respondents, however, said they don't feel they're allowed to speak freely about their work in the media, and 86 per cent believe they would face retaliation if they went public with information about harm to public health, safety or the environment.

Corbett noted the 2006 Government of Canada communications policy states it must provide the public with "timely, accurate, clear, objective and complete information" about its policies, services and programs.

"Whether by implicit policy or explicit action, there has been silencing and it continues," Corbett said.

But the survey was equally damning in its assessment of the government's use of scientific research.

Just 21 per cent of respondents said Environment Canada uses the best climate change evidence available to make policy, while only 29 per cent agreed Natural Resources Canada does so.

Over at Fisheries and Oceans, 86 per cent of respondents said they felt changes to the Fisheries Act will hamper Canada's ability to protect fish and their habitat.

Peter Bleyer of PIPSC said that when the 55,000-member union does broader membership surveys, they typically get a fraction of the response rate the science survey achieved.

Anecdotally, respondents said the muzzling of science has become worse or was never as bad before the Conservatives came to power. But that's really not the issue, said Bleyer.

"Whether or not this problem existed before, it is a problem," said Bleyer. "It's a potential threat to all Canadians. We need to fix it."

Treasury Board President Tony Clement's office did not respond to a request for comment.