Mr. Haines is a historian at the National Reconnaissance office, the intelligence agency that builds and runs the naiton's spy statellites.

The admission of Federal deception on the issue appears to be a first, experts said in interviews.

''It's very significant,'' said Richard Hall, chairman of the Fund for U.F.O. Research, a group in Washington. ''Certainly they've lied about not having any interest in the subject. But I don't know of any other deception like this.''

John E. Pike, head of space policy at the Federation of American Scientists, also based in Washington, said the admission raised questions about other Federal cover-ups involving U.F.O.'s.

''The flying-saucer community is definitely onto something,'' in charging that the military is hiding something, Mr. Pike said.

There are two schools of thought on the nature of such a cover-up. One, from Mr. Pike and other aerospace experts, holds that many sightings over the decades involved secret Federal projects featuring advanced aircraft and reconnaissance missions. The new admission strengthens that view.

The other school holds that the Government has come into possession of extraterrestrial craft and beings and is hiding them from the public, partly to avoid causing panic. That view was celebrated last month on the 50th anniversary of an incident in Roswell, N.M., in which conspiracy theorists say a saucer crashed to Earth and was seized by the Government.

The deceptions about the spy flights were issued in some of the tensest days of the cold war. The Soviet Union exploded its first hydrogen bomb in 1955, the year that the U-2 flew for the first time.