It was an historic and sensitive trip, planned with precision - and in secret - at a time of high tension in the cold war.

But Mrs Thatcher's visit to west Berlin in October 1982, during which she made a stand against communism in front of the Berlin wall, was a security fiasco from start to finish.

According to files from the archives of east Germany's notorious secret police, the Stasi, communist spies secretly photographed and closely observed Mrs Thatcher throughout her trip after managing to get hold of an advance copy of her itinerary.

Mrs Thatcher travelled to the border with east Berlin, close to Berlin's most famous monument, the Brandenburg Gate, and climbed up a viewing platform overlooking the Berlin wall.

The documents show that east German spies secretly photographed Mrs Thatcher from the other side as she travelled standing in an open-topped Land Rover, with Germany's new chancellor Helmut Kohl.

But the documents note that eight British military policemen strayed into east German territory just before Mrs Thatcher's arrival when they turned up to inspect the viewing platform next to the wall.

The policemen inadvertently walked 300 metres into the eastern zone, apparently, the Stasi concluded, while looking for bombs in an area they wrongly thought was no man's land.

"There was an incident," Lieutenant Greulich, the head of the Stasi's intelligence division wrote."Between 12.20 and 12.50 the eight members of the British military police infringed the forbidden zone.

"The members of the British military police were armed," he added.

Yesterday Anthony Glees, a historian who has written extensively on the Stasi, said the documents showed how the government consistently underestimated the extent of the Stasi's activity throughout the 1980s, during a crucial period in the cold war.

"They indicate potentially very serious security lapses. This was a time of huge international tension after Nato's rearmament. Helmut Kohl had only recently become chancellor. The whole Soviet bloc regarded the west as a mortal enemy and the Iron Lady was at the zenith of her power. She was an extremely powerful person coming to Berlin, and the east Germans knew where she was going to be and when."

Although communist east Germany had no track record of assassinating foreign heads of state, it had well documented links with terror groups active in Europe and the Middle East. The Stasi also kept a close watch on the activities of the IRA, who nearly killed Mrs Thatcher during the Brighton bombing, two years after her Berlin trip. Other documents show that the Stasi broadly sympathised with the IRA, whom they regarded as ideological comrades engaged in the same struggle against colonialism.

Prof Glees said communist east Germany may have been domestically repressive but it did not try to kill its foreign enemies. He added:"They were not involved in what were known in the trade as 'wet jobs'. But they did support terrorist acts by others."

Mrs Thatcher described the wall during her trip to Berlin as"far worse than I had ever imagined". She also expressed her"unshakeable determination" that Britain would defend west Berlin, and predicted that"one day there will be freedom on the other side of the wall".

A short biography appended to the Stasi's file on Mrs Thatcher describes her as"hard-working and intelligent". It goes, on, however, to say she has"another side" as well. She could be" cold, ambitious and moralistic".