By Andrea Shalal-Esa

WASHINGTON (Reuters) - The cost of each Lockheed Martin Corp (LMT.N: Quote, Profile, Research) F-35 fighter jet has soared 38 percent since 2001, but costs have been virtually unchanged over the past year, according to a Pentagon document obtained by Reuters on Tuesday.

The F-35 program is the most expensive weapons program in U.S. history. The Pentagon is developing versions of the fighter for the Air Force, Marine Corps and Navy, and to sell to other countries.

The cost of each F-35 rose 38.01 percent to $69.3 million per plane at the end of 2007 from $50.2 million in October 2001, when the development program began; but that was an increase of just 0.25 percent from December 2006, according to a copy of the draft document.

The overall program cost remained stable at $299 billion over the past year, after a 29 percent jump from 2001 to 2006 due to schedule changes, inflation indices, material cost increases and higher labor rates, according to the document.

Estimated operation and support costs for the F-35 over the life of the program, though, rose 17.5 percent to $764 billion from the December 2006 estimate of $650 billion, it showed. The document did not explain the increase, but the cost of jet fuel zoomed higher during 2007.

The overall program cost increase from 2001 may force the Pentagon to notify Congress but falls short of a congressional threshold triggering a Pentagon national security review that could lead to program termination.

Stable costs would be good news for Lockheed and Northrop Grumman Corp (NOC.N: Quote, Profile, Research), a key subcontractor on the program, said Virginia-based defense consultant Jim McAleese.

He said F-35 officials were now reviewing costs and seeking an independent estimate. Any notification to Congress would likely be delayed until September, he added.