(Adds Raytheon statement)

WASHINGTON, Oct 21 (Reuters) - Northrop Grumman Corp on Tuesday said it had filed a formal protest with the U.S. Government Accountability Office against the U.S. Air Force's decision to pick Raytheon Co to develop a next-generation, long-range radar system.

Raytheon beat out Northrop and Lockheed Martin Corp to develop a replacement for the Air Force's current TPS-75 radar, which has been in service since the late 1960s.

The value of the Three Dimensional Expeditionary Long-Range Radar (3DELRR) contract is limited now, but could rise sharply in coming years, given the Air Force's plan to buy 30 of the new systems in coming years, plus orders from foreign militaries.

Northrop spokesman Randy Belote said only that the company had protested the Air Force decision, but declined to provide further comment about the challenge.

Air Force officials said they briefed the losing bidders about their decision last week.

Spokeswoman Patty Welsh from Hanscom Air Force Base in Massachusetts said the Air Force issued a stop-work order for the program after the protest was filed on Tuesday.

Raytheon said it would work closely with the Air Force to mitigate any resulting delays once the issue was resolved.

"The Air Force ran a very tough but very fair competition, and selected the world leader in radar to build 3DELRR," said Raytheon spokesman Mike Doble. "We remain confident in our solution and we're eager to move forward and deliver this much needed 3DELRR capability to the U.S. and its friends and allies."

Lockheed officials said they were still reviewing the briefing they received from the Air Force about the contract award to Raytheon.

Industry officials and analysts say the stakes are particularly high for Northrop Grumman, which lacks the large aircraft and ship programs that undergird Lockheed's more diverse portfolio.

Northrop Grumman has lost out to Raytheon on several major radar contracts recently, including the Navy's next-generation jammer and a new air and missile defense radar, although it still makes critical radars for the F-22 in the tactical market.

(Reporting by Andrea Shalal; Editing by Meredith Mazzilli, Bernard Orr and Lisa Shumaker)