Study Flags Risks to US Missile Defense Program

* GAO warns of potential cost overruns, delays

* US cited Iran threat in rollout of missile defense plan

WASHINGTON (Reuters) - U.S. President Barack Obama's planned missile defense system for Europe could prove unreliable and risks delays and cost over-runs, congressional investigators said in a new report.

The United States announced plans last year to integrate sea- and land-based missile defenses in Europe, a program referred to as the "European Phased Adaptive Approach."

U.S. anti-ballistic missile systems are meant to cover Europe by around 2018, and NATO allies in Europe agreed last month to bolster the missile shield.

But the Government Accountability Office (GAO), the investigative arm of Congress, warned in a report dated Tuesday that there was limited visibility into the costs and timetable for the program.

It warned that one of the consequences could be "going into production before fully demonstrating system performance, leading to rework, cost increases, delays, and uncertainties about delivered capabilities."

Key components for the shield are updated Raytheon Co Standard Missile-3 missile interceptors and Lockheed Martin Corp's Aegis weapons system.

The United States has already spent more than $10 billion a year on a range of missile defense programs in recent years, but critics say those programs are still far from reliable.

A test of the sole U.S. defense against long-range ballistic missiles failed last week, the second failure in a row involving the system managed by Boeing Co .

The miss brought the so-called ground-based midcourse defense's batting record to eight intercepts out of 15 tries, as reckoned by the Missile Defense Agency.

The United States says its European missile defense efforts are designed to defend against Iranian short- and medium-range missiles that could be tipped with chemical, biological or nuclear warheads.

(Reporting by Phil Stewart; Editing by Cynthia Osterman)