WASHINGTON — Following back-to-back flight test failures of the nation’s primary strategic missile shield, the U.S. Missile Defense Agency (MDA) in February canceled its plans to buy long-range target missiles this year and reduced by 20 percent the total number of missile defense tests it plans to conduct during the next few years.

The MDA maintains an Integrated Master Test Plan that details all of the ballistic missile defense flight and ground tests that the agency plans to carry out through 2016. The MDA in February revised this test plan, reducing the total number of tests from 150 to 120, agency spokesman Rick Lehner said March 2. However, the tests will be made more comprehensive and accomplish 97 percent of the objectives laid out in the previous plan, he said.

The spending profile for tests and targets was reduced in the 2012 budget request MDA sent to Congress Feb. 14. The agency requested $1.07 billion for tests and targets next year and plans to spend a total of $3.54 billion from 2012 through 2015 — some $211 million less than anticipated for the same period in its previous budget request.

The agency is currently reviewing a Dec. 15 flight test failure of the Ground-based Midcourse Defense (GMD) system, the nation’s primary shield against  long-range missile strikes. A target missile was launched from the Kwajalein Atoll in the Pacific Ocean, and a ground-based interceptor that launched from Vandenberg Air Force Base, Calif., failed to destroy it.

The MDA initially reported the system’s Sea-Based X-band radar, which was being used for the first time to detect a target missile launch in a GMD test, failed to perform as expected. A failure review board is still assessing the problems that occurred in the test known as FTG-06a. The previous GMD test in January 2010 was also unsuccessful.

The GMD system has recorded eight intercepts in 15 flight tests since 2001.

The MDA previously planned to conduct the next GMD test this year, but that test will take place no sooner than 2012 pending the investigation of the most recent test failure, Lehner said.

The GMD system, developed by Boeing Defense, Space and Security of St. Louis, has not  been tested against the type of long-range missiles it is intended to defeat. The MDA last year initiated its first procurement of ICBM-class target missiles, and a final request for proposals was expected to be issued this month. But with the modification of the Integrated Master Test Plan, the agency on Feb. 16 canceled the procurement, according to documents posted on the Federal Business Opportunities website. A competition to build the targets will be held at a future date, it said.

The MDA had planned to test the GMD system against ICBM targets in late 2014, 2015 and 2016. Though the GMD test schedule for the years ahead is still being determined, the agency still expects to begin testing against ICBM-class targets in 2014, Lehner said.

The agency is now holding a competition to continue development and sustainment of the GMD system for the decade ahead. Incumbent Boeing is teamed with Los Angeles-based Northrop Grumman Corp., and they seek to fend off a challenge from a partnership between Lockheed Martin Corp. of Bethesda, Md., and Raytheon Co. of Waltham, Mass. The MDA is expected to announce a winner May 31, according to documents posted on the Federal Business Opportunities website.

Meanwhile, the MDA on March 1 issued a $72 million contract modification to extend Boeing’s GMD work through August.