The first flight of the space shuttle from the West Coast has been postponed from mid-March to mid-July, the Air Force and the National Aeronautics and Space Administration said yesterday.

In a joint statement, the Air Force and NASA blamed the delay on minor construction snags at the $2.5 billion site at California's Vandenberg Air Force Base and on the realization that more extensive preparation and training is needed for the West Coast launch crews. The delay means the first shuttle flight from Vandenberg is now nine months behind schedule.

By moving to California from Florida's Cape Canaveral, the spaceliner can be programmed to fly paths that take it in a north-south direction, over the Earth's poles.

Florida launches cannot use polar routes because such flights would take the shuttle over heavily populated areas of the East Coast, posing added dangers if the shuttle aborted its flight or crashed.

"There are no major problems at the Vandenberg site," Air Force Undersecretary Edward C. (Pete) Aldridge Jr. said yesterday. "We have repeatedly stated that safety and quality would not be sacrificed for schedule."

Said NASA Associate Administrator Jesse W. Moore: "NASA concurs completely with the Air Force regarding Vandenberg. Our first commitment is to the safety of the crew and the reliability of the vehicle and launch systems."

The California shuttle launch was originally scheduled for Oct. 15, then set back to Jan. 29 and postponed again to March 20.

The first delays were caused by high winds at the Vandenberg launch site that hampered construction and by underestimates of construction time.