Washington  - The US Defence Department on Wednesday formally reopened bidding for the contract to build the next generational of Air Force tanker refuellers, a spokesman said.

The contract was initially won by the Northrop Grumman-EADS partnership but a congressional oversight agency upheld a protest by rival Boeing, effectively forcing the Pentagon to rehold the competition for the 35-billion-dollar contract.

The Pentagon on Wednesday formally asked the companies to resubmit proposals to build the 179 KC-X aerial refuellers, spokesman Chris Isleib said.

The Government Accountability Office (GAO), an investigative arm of Congress, ruled in a June 18 report that the Air Force made critical errors in awarding the contract to Northrop and the European Aeronautic Defence and Space Company.

The GAO concluded the Air Force overlooked key aspects of the Boeing proposal that could have tilted the competition to the American aerospace giant's advantage and urged the Pentagon to reopen the bidding.

Defence Secretary Robert Gates announced July 9 that he had accepted GAO's findings and planned to restage the competition in an expedited process, citing the need to quickly replace the Air Force's aging fleet of tankers. (dpa)