The Stasi earned a frightening reputation for thoroughness as East Germany's secret police.

At its height it employed 85,000 full-time officers, had records on five million East German citizens - one third of the entire population - and had several hundred thousand informers.

Mark Almond, lecturer in modern history at Oxford University, said they would have wasted no time in recruiting a student like Robin Pearson, who was already a member of the communist youth movement when he visited Leipzig in East Germany.

He said: "The Stasi, like any of the communist bloc secret services, had a voracious appetite for recruiting anyone from a Western Nato country.

"I am sure that any British person who spent some time as a student or researcher in East Germany must have been assessed as a potential ideological ally or perhaps somebody who for personal weaknesses or bribery would be willing to work for them."

If, like Robin Pearson, that student subsequently began an academic career, The Stasi would have known it had roped in a potential goldmine.

Mr Almond said: "Somebody who worked in a university for instance, might after all teach somebody who went into politics, went into the army or was a scientist who could be valuable, not just to the East German Stasi, but for all the former communist bloc secret services."

Willy Brandt: Resigned over aide's Stasi link

The Stasi, or Ministry for State Security (Staatsicherheit), was created in 1950 by German communists in the Soviet zone of occupied Germany soon after World War II and was disbanded with the fall of the Berlin Wall in 1989.

Responsible for both domestic political surveillance and foreign espionage, it soon earned a reputation for effectiveness. It extensively penetrated West Germany's governing circles and military and intelligence services.

The discovery in April 1974 that G�nter Guillaume, a top aide to West German Chancellor Willy Brandt, was in fact an East German spy led Mr Brandt's resignation two weeks later.

Under the direction of Erich Mielke, the Stasi established a reputation for ruthlessness, with dissidents being imprisoned and tortured for such "crimes" as trying to leave the country, or telling political jokes.

Erich Mielke: Stasi chief

The notorious H�hensch�nhausen prison in the north-east suburbs of Berlin, where many of the dissidents were kept, is now a memorial to those who were detained there.

Although it is no longer a prison, it has lost nothing of its power to scare the visitor arriving at its steel gates.

Former inmates guide visitors around the underground cells, and describe how they were kept, isolated and disoriented, knowing nothing of what was going on in the outside world.

One of the abiding images of German reunification is Germans ransacking the Stasi buildings in a bid to remove all traces of the hated secret police's records.

However many files were left intact and allegedly among them, were those on Hull University lecturer, Dr Robin Pearson.

It is these files which BBC journalist David Rose has used for the series The Spying Game and allegations against Dr Pearson that he worked for The Stasi.

It appears the efficient filers of the Stasi can be thanked for uncovering yet another British spy.