The official also rejected calls to reopen the barrels that contain the wreckage of Gagarin's plane, the Novosti news agency reported.
Gagarin and a flight instructor, Vladimir Seryoghin, were both killed during a routine training flight March 27, 1968. The cosmonaut was retraining as a fighter pilot.
The crash, seven years after Gagarin's historic trip into space, has spawned a number of conspiracy theories. One is that Leonid Brezhnev, who then headed the Soviet Union, had him killed out of jealousy, while a Finnish group has suggested that Gagarin never went into space and Soviet officials wanted him dead to preserve the secret.
A 1986 inquest suggested that a jet's afterburners caused turbulence that brought Gagarin's plane down.
Gagarin and his instructor were buried in the Kremlin wall. A town near Smolensk was named in the cosmonaut's honor.