WASHINGTON --

The Defense Department will push back its decision on a $35 billion tanker contract to the next administration, delaying again the hotly disputed competition between Boeing and Northrop Grumman to replace the Air Force's aging aerial refueling fleet.

Defense Secretary Robert Gates said Wednesday he decided to cancel the current round of bidding on the plane - a competition that has stretched seven years - because the Pentagon's plan to award the contract by the end of the year no longer seemed possible given the complexity of the project and the rancor between the two companies. He said a delay would provide a "cooling off" period.

"We can no longer complete a competition that would be viewed as fair and objective in this highly charged environment," Gates said in a statement.

The Pentagon was expected to release its formal set of guidelines as early as Aug. 15 for the last round of bidding for the right to build 179 new planes, but that target has continued to slip. The deal would be the first phase of what could eventually be a much larger fleet and much more lucrative contract. Many of the Air Force's current airborne tankers are around 50 years old.

Northrop Grumman Corp., which has partnered with Airbus parent European Aeronautic Defense and Space Co., was awarded the contract earlier this year, but a subsequent Government Accountability Office review found major flaws with the way it was awarded. The Pentagon reopened the bidding in August.

Boeing said the new competition was unfairly tilted against its smaller plane, a version of the commercial 767 jet. It threatened to back out of the bidding if the Pentagon did not give it more time to come up with a new proposal.

"They didn't have enough time to do it right," Boeing supporter Rep. Norman Dicks, D-Wash., said in an interview.