The court found that the agency, the Federal Motor Carrier Safety Administration, a unit of the Department of Transportation, had ignored the results of a database it commissioned to catalog more than 50,000 truck accidents from 1991 to 2002. Using the data, the study extrapolated that the risk of fatigue-related accidents would be substantially higher in the extra hours of service allowed by the new rules.

“F.M.C.S.A. failed to provide an adequate explanation for its decision to adopt the 11-hour daily driving limit,” the court said.

The new rules had been adopted after heavy lobbying by politically connected leaders of the trucking industry. The changes were part of the broader strategy by the Bush administration to reduce regulations on businesses.

Safety experts and insurance analysts challenged the changes. They said longer driving hours have contributed to the high number of truck accidents. About 100 people die each week in truck-related accidents, making trucking America’s most treacherous industry as measured by overall deaths and injuries.

Supporters of the loosened standards say they have made it faster and cheaper to move goods across the country. They say the changes promote safety because shorter hours would force the industry to put more drivers with little experience behind the wheel. And they note that the fatality rate, or the number of deaths per miles traveled, has continued a long decline.

Still, the fatality rate for truck-related accidents remains nearly double that involving only cars. And the Bush administration has repeatedly missed its own targets for reducing the number of fatalities from truck accidents.

The decision today came in a case filed by Public Citizen, a consumer advocacy group. It was the third time in three years that the courts have been critical of the motor carrier agency.

A different appeals panel criticized the agency in December 2005 for failing to issue adequate rules for the training of drivers, saying the agency had ignored its own studies on the need for more comprehensive training.

And in 2004, a third panel of the appeals court struck down virtually identical new hours of service rules as the ones at issue in Tuesday’s decision, saying that they had been “arbitrary and capricious.”

After Congress, at the urging of the Bush administration and the trucking industry, intervened to block the enforcement of the 2004 court order, the motor carrier agency issued the 2005 rules. At the time, the agency said it had addressed the concerns raised by the appeals court’s 2004 decision.

In a regulatory impact analysis accompanying the 2005 changes, the agency concluded that the economic costs to the industry of tightening the hours of service rules, as consumer groups had proposed, outweighed the safety benefits.

But the court said today that analysis was flawed. The opinion was written by Judge Merrick B. Garland and signed by Chief Judge Douglas H. Ginsburg and Judge Karen Lecraft Henderson.

Safety groups hailed Tuesday’s ruling and said the court had confirmed their view that the agency had failed to adequately justify relaxing the rules.

“The court is saying once again, no,” said Jacqueline S. Gillan, vice president of Advocates for Highway and Auto Safety, an alliance of consumer, health and insurance organizations. “For three and a half years this agency has tried every which way to defend a rule that would result in longer consecutive driver hours and longer total work hours. This has a dramatic dangerous impact on the lives of truck drivers and on the lives of everyone sharing the roads with trucks. And once again the court has said, ‘No, you cannot go ahead with a rule when it violates the law and you clearly have not justified it.’ ”

The agency would not say whether it would appeal the decision or seek a stay of the court’s order, which is set to go into effect in September.

“We are analyzing the decision issued today to understand the court’s findings as well as determine the agency’s next steps to prevent driver fatigue, ensure safe and efficient motor carrier operations and save lives,” said a statement issued by the agency.

The American Trucking Associations, which defended the changes to the rules in the proceeding, said they would ask the court to stay its ruling to give the agency time to provide a better justification of the changes.