By Andrea Shalal-Esa

WASHINGTON (Reuters) - Lockheed Martin Corp (LMT.N: Quote, Profile, Research) said on Monday it filed a protest against the U.S. Navy's decision last month to award a multibillion-dollar contract for an unmanned patrol plane to Northrop Grumman Corp (NOC.N: Quote, Profile, Research).

Lockheed said it filed the protest with the U.S. Government Accountability Office (GAO) after receiving a briefing from the Navy showing that the Lockheed proposal met the Navy's technical requirements and cost significantly less, said spokeswoman Tierney Helmers.

Northrop beat out Lockheed and Boeing Co (BA.N: Quote, Profile, Research) to win the initial contract valued at $1.16 billion with its proposal based on the high-altitude unmanned Global Hawk. The deal is worth at least $3.74 billion in the longer term, the Navy said.

Bethesda, Maryland-based Lockheed was involved in a high-profile protest against a $15 billion Air Force helicopter contract awarded to Boeing in 2006. In that case, the GAO twice upheld protests filed by Lockheed and United Technologies Corp (UTX.N: Quote, Profile, Research) unit Sikorsky Aircraft. Boeing is now protesting a $35 billion Air Force contract it lost to Northrop in February.

The congressional agency, which reviews federal contract disputes, confirmed it had received Lockheed's protest and said a decision was expected by Aug. 13. A spokesman said the protest would trigger a stop-work order from the Navy.

Asked about the protest, the Navy said it was confident the acquisition process had been fair, and that Northrop's proposal provided the "best overall value."

"The government is confident the source selection process considered all proposals fairly, equally and in full compliance with the stringent Federal Acquisition Regulations," Navy spokesman Lt. Cmdr. John Schofield said.

Northrop said it was confident the GAO would uphold the award, and it would work with the Navy to ensure the program remained on cost and on schedule.