RAF jets were scrambled to investigate UFOs spotted on radar systems about 200 times a year during the Cold War, the secret files show.
But after the collapse of the Soviet Union in 1991 the number of reported sightings fell to zero as Moscow stopped sending anti-submarine aircraft and spy planes into British airspace.
The RAF, however, continued to scramble aircraft in missions against known targets, including hijacked airliners, suspected drug smugglers and jets launched from a Russian aircraft carrier, the National Archive files show.
The information is revealed in a Ministry of Defence briefing note for a response to a July 1996 Parliamentary Question from Martin Redmond, then Labour MP for Don Valley in South Yorkshire.
He had asked how many times RAF aircraft had been scrambled or diverted from tasks to investigate "uncorrelated targets" picked up on radar.
The MoD replied that over the past five years there had been no incidents.
But further details included in a background note, said: "Prior to the demise of the former Soviet Union, aircraft were scrambled some 200 times annually to intercept and investigate uncorrelated tracks penetrating the UK air defence region from the north.
"These invariably proved to be anti-submarine or long range reconnaissance aircraft of the then-Soviet air force, some of which had already been intercepted by adjacent air defence systems or intelligence sources."
The last scramble of this kind took place in September 1991.
The RAF later occasionally helped customs officers monitor activity in certain areas to detect drug running.