A new federal report says the Pentagon used faulty data to defend its plan to transfer thousands of defense workers to a massive Alexandria office complex, and local leaders are being urged to use the report in a legal effort to block the move.

The report by the Defense Department’s inspector general finds fault with the 2008 Pentagon study that minimized the impact the relocation would have on the environment and traffic on already congested Interstate 395.

“This is what we’ve been saying for the last four years,” said U.S. Rep. James P. Moran (D-Va.), who has opposed the move. “Now we have the inspector general saying that their justification was faulty and was deficient.”

Moran said he had forwarded the report, available Wednesday, to officials in Alexandria and Fairfax County with the recommendation that they use it to file suit in U.S. District Court asking for an injunction to block the move.

The Pentagon plans to relocate 6,400 defense workers to the Mark Center complex on Seminary Road, adjacent to I-395, on Sept. 15. Almost all employees would be moved from Crystal City, where they have access to the Metro system, to a location that would require them to commute by car.

“We can’t add that on top of the 200,000 commuters who use 395 every day,” Moran said.

Sharon Bulova (D), chairman of the Fairfax County Board of Supervisors, had not read the 202-page report but said she would do so with an eye toward options.

“Waging a lawsuit would take money and resources,” Bulova said. “If we were to go down that road, we need to first assess our chances for being successful.”

Alexandria Mayor William D. Euille (D), who had not read it either, said the city would “have to weigh our options and consider our next move.”

“Obviously a mistake was made here. The bad news is that they may not do anything about it and keep moving forward,” Euille said.

The inspector general said that traffic studies relied upon by the Army “did not adequately address existing and projected peak hour volumes.” The Army concluded the move would have no significant impact on traffic.

The inspector general said the Army should do a more thorough assessment of the potential traffic congestion and additional review of the environmental impact.

The move to the Mark Center has been opposed by both Virginia U.S. senators and Northern Virginia’s two Democratic congressmen.

The workforce relocation is part of the nationwide Base Realignment and Closure (BRAC) process, which will have an enormous effect on the Washington area, with major expansions at Fort Meade, Fort Belvoir and the National Naval Medical Center in Bethesda.

Members of Maryland’s congressional delegation have joined their Virginia counterparts in complaining that the Pentagon made its decisions without regard for the impact on traffic and without providing the money needed for highway improvements. In most circumstances, workers are being transferred from offices with access to mass transit to locations that lack it.

A report issued by the National Academy of Sciences in February said the moves put an unfair financial burden on state and local governments. It recommended that Congress consider a one-time allocation of new money or reprogram unused stimulus money to pay for transportation improvements.

Congress responded by including $300 million in the 2011 budget deal to expand roads around Fort Belvoir and the National Naval Medical Center. It didn’t provide money for road improvements at the Mark Center.

Last week, however, the Pentagon said it would make $20 million available for highway improvements around the Alexandria office complex .

The Pentagon’s commitment is to be used for ramp improvements, intersections and crosswalks at the Mark Center. But a required environmental study on the ramp will delay the construction start until next year.

Under its guidelines, the Pentagon is not required to help pay for transportation improvements surrounding a military base slated for growth unless its actions would cause congestion to double. For a region already burdened with some of the nation’s worst traffic problems, that was an impossible and undesirable standard to meet.