WASHINGTON - The head of the U.S. spy satellite agency, the National Reconnaissance Office, has announced his resignation, a spokesman said on Thursday.

Scott Large sent a memo to NRO employees on Wednesday announcing his decision, which will be effective on April 18, NRO spokesman Rick Oborn said.

His announcement came just a day after the U.S. government said it would buy expensive new spy satellites and order more imagery from two commercial providers in a plan which analysts estimate will cost around $10 billion.

Oborn said Large had given no specific reason for his departure, saying he had made a personal decision.

His resignation was first reported by the website dodbuzz.com, which reports on the Department of Defense.

The Obama administration approved the new spy satellite plan despite strong criticism from Congress.

The program will replace one that was initially awarded to Boeing Co, but was partially canceled three years ago when its costs soared billions of dollars over budget.

Sen. Christopher Bond, the top Republican on the Senate Select Committee on Intelligence, told Dennis Blair, the director of national intelligence, in a letter dated March 16, that the new plan had zero chance of delivering satellites on schedule and would cost too much.

A former government official, who spoke anonymously given the classified nature of the program, said the contract would likely go to Lockheed Martin Corp, particularly given concerns about Boeing's work on the earlier program.

Large was appointed NRO director in October 2007. The director is chosen by the defense secretary with the agreement of the director of national intelligence.

Central Intelligence Agency