The Government Accountability Office (GAO) released an extended explanation of its decision last week to uphold Boeing's protest against the contract award to Northrop Grumman Corp and Europe's EADS .

GAO said the Air Force competition to build refueling tankers was seriously flawed and unfairly favored the eventual winners,

Without those errors, losing bidder Boeing Co would have had a "substantial chance" of winning the contract, said the GAO, an arm of Congress that reviews federal contract bidding disputes.

In its 22,000 word document, redacted for public viewing, the GAO said the Air Force conducted "misleading and unequal discussions" with Boeing.

For example, it told Boeing it had fully satisfied an operation utility requirement covering things like maneuverability and runway operations, but later deciding it had only partially met the objective, without telling Boeing.

The contract was awarded on February 29 to a team made up of Northrop and EADS, the corporate parent of Boeing's passenger-jet rival Airbus.

Boeing protested the award on March 11, claiming it was handled improperly and unfairly favored the Northrop-led team.

The GAO upheld Boeing's protest on June 18, specifically relating to a number of Air Force errors, but said it also denied many of Boeing's challenges to the award.

According to the GAO document published on Wednesday, the Air Force made an error in crediting the Northrop team for exceeding a key aerial refueling objective, directly against the Air Force's own proposal that "no consideration will be provided for exceeding" key performance parameters.

It also made errors in assessing life-cycle costs of both teams' proposals, which made Boeing's appear more expensive, the GAO said.

The GAO has recommended that the Air Force reopen discussions with the bidding teams and rerun the competition. The Air Force, which has 60 days to respond to the initial GAO decision, is not bound to follow the GAO's recommendations.

The GAO emphasized that its ruling should not be interpreted as support for either team's bid.

"This decision should not be read to reflect a view as to the merits of the firms' respective aircraft," the GAO document said.

Boeing said the GAO document further validated the company's decision to protest the contract award and confirmed the strength of its tanker proposal.

"It is clear the award was the result of a flawed process," said Mark McGraw, Boeing's head of tanker programs.

Northrop said the document made clear the GAO's issues with the contract do not reflect on the tankers' capabilities.

"In fact, in several areas key to the selection decision the GAO found no basis to object to the Air Force evaluation," said Northrop's vice president for air mobility systems, Paul Meyer, in a statement.

(Reporting by Bill Rigby; Editing by Tim Dobbyn)