A U.S. government agency withheld research it compiled in 2002 and 2003 warning of the dangers of cellphone use while driving, and its former head says political interference buried the information.

The Center for Auto Safety, a consumer advocacy group, made the documents public for the first time on Tuesday after filing a Freedom of Information Act request and lawsuit.

The research, compiled by the Department of Transportation's National Highway Traffic Safety Administration, cites pages of previous studies that prove, for example, that drivers experience a "general delay in information processing and degradations in driving performance" when talking on a cellphone.

The report, entitled, "Status Summary: Using Wireless Communication Devices While Driving," also pointed to a number of studies that found a dramatically increased risk of a car crash for cellphone users.

The NHTSA said it estimates that cellphone use caused more than 900 fatalities in the U.S. in 2002, and hundreds of thousands of car accidents.

"The experimental data reveal observable degradations in driver behavior and performance and changes in risk-taking and decision-making behaviors when using both hand-held and hands-free mobile phones, and the nature of those degradations and changes are symptomatic of potential safety-related problems," wrote the researchers.

Dr. Jeffrey Runge, who was then the head of the NHTSA, told the New York Times that the agency decided not to make the documents public due to political pressure.

According to Runge, Transportation Department officials told him the research should not be used to lobby states to enact specific legislation. If the agency was seen to be lobbying the states, it could lose billions of dollars in funding, officials warned Runge.

Runge said he wanted to inform the general public about the dangers of talking and driving.

"Based on the research, there was a possibility of this becoming a really big problem," Runge said.

But "my advisers upstairs said we should not poke a finger in the eye of the appropriations committee."

Runge speculated that the House Appropriations Committee feared alienating voters who aren't in favour of laws restricting what they can do in their own cars, as wells as perhaps the cellphone industry.

In addition to withholding the report, the agency also held back a letter it wrote for then-Transportation Secretary Norman Y. Mineta to send to governors warning against driver use of both hand-held cellphones and hands-free devices, such as ear pieces and headsets.

"We recommend that drivers not use these devices when driving, except in an emergency," the letter stated. "Moreover, we are convinced that legislation forbidding the use of handheld cellphones while driving will not be effective since it will not address the problem. In fact, such legislation may erroneously imply that hands-free phones are safe to use while driving."

Mineta, who left his post as transportation secretary in 2006, told the Times he knew nothing of the research.

A spokesperson for the CAS was not immediately available for comment.

The issue of cellphone use while driving is becoming an increasingly hot-button issue in Canada.

In April, Ontario passed a law that bans using hand-held devices to talk, email or send text messages while driving. The new rule, which does not apply to hands-free devices such as Bluetooths, will come into effect his fall.

Ontario was the fourth province to enact a ban, after Newfoundland and Labrador, Quebec and Nova Scotia.