Many other fascinating UFO-related documents can now be seen at the National Archives.
Transcript follows. Many thanks to Martin.
SECRETARY OF STATE FOR AIRResponse
What does all this stuff about flying saucers amount to? What can it mean? What is the truth? Let me have a report at your convenience.
The various reports about unidentified flying objects, described by the Press as "flying saucers", were the subject of a full Intelligence study in 1951. The conclusions reached (based upon William of Occam's Razor) were that all the incidents reported could be explained by one or other of the following causes:-
(a) Known astronomical or meteorological phenomena
(b) Mistaken identification of conventional aircraft, balloons, birds, etc.
(c) Optical illusions and psychological delusions
(d) Deliberate hoaxes.
2. The Americans, who carried out a similar investigation in 1948/9, reached a similar conclusion.
3. Nothing has happened since 1951 to make the Air Staff change their opinion, and, to judge from recent Press statements, the same is true in America.
4. I am sending a copy of this to Lord Cherwell.
9th August, 1952.