The news was almost as unexpected as the decision by the Air Force earlier this year to award a $35 billion tanker deal not to The Boeing Co. but to a team that included the parent of Airbus.

In a decision that many experts had predicted would be highly unusual, the Government Accountability Office sided Wednesday with Boeing in its protest of the tanker decision.

"Our review of the record led us to conclude that the Air Force made a number of significant errors that could have affected the outcome of what was a close competition between Boeing and Northrop Grumman," the GAO said. "We therefore sustain Boeing's protest."

The GAO, which is the investigative arm of Congress, recommended that the Air Force reopen talks with Boeing and the team of Northrop and European Aeronautic Defense and Space Co. and hold a new tanker selection process. The congressional agency used particularly strong language in saying that the Air Force made mistakes in seven critical areas, according to a military expert.

"I can't imagine that with this many errors identified by the GAO that the Air Force will be able to escape holding a new competition," said Loren Thompson, a noted military analyst with the Lexington Institute, a public policy research group in Arlington, Va.

The GAO announcement was greeted by cheers from Boeing workers on the 767 line in Everett, where any Boeing tankers for the Air Force would be built. The 767 jetliner is being replaced by the 787 Dreamliner in Boeing's commercial lineup, and without the tanker contract, the assembly line would be shut down in a few years when the last of those 767 passenger planes and freighters are built.