The fall of the Berlin Wall in November 1989 was hailed in the West as a seminal moment in its “struggle” against Communism and sparked a wave of euphoria. But, it has now emerged, that behind those euphoric public pronouncements there were deep anxieties in most European capitals, especially in London and Paris.

Indeed, neither Margaret Thatcher, the then British Prime Minister, nor French President Francois Mitterrand wanted the wall to come down as they feared that a unified Germany would be a “threat” to European security.

Mrs. Thatcher was so concerned that two months before the fall of the wall she travelled to Moscow to plead with Soviet leader Mikhail Gorbachev to intervene and stop the break-up of East Germany. In Paris, meanwhile, Mr. Mitterrand was deploying Gallic humour to voice his concern saying: “I like Germany so much I would prefer to have two of them.”

Even at the time, it was known that there was nervousness in Europe at the prospect of a united Germany (in her memoirs The Downing Street Years published in 1993 Mrs. Thatcher recalled her own reservations) but the extent of paranoia is revealed for the first time in confidential Kremlin documents extracts from which were published in The Times last week.

These are among some 1,000 official papers that Pavel Stroilov, a young researcher in the Gorbachev Foundation smuggled out, when he moved to Britain a few years ago.

The newspaper said they showed that at a luncheon meeting with Mr. Gorbachev in Moscow in September 1989, Mrs. Thatcher voiced her deep “concern” at the turmoil in East Germany and warned that a change in post-war borders would undermine European security.

“We do not want a united Germany. This would lead to a change to postwar borders, and we cannot allow that because such a development would undermine the stability of the whole international situation and could endanger our security,” she told Mr. Gorbachev.

Even after the fall of the wall, London and Paris continued their efforts to stop reunification of Germany. Barely weeks after the wall came down, Mr. Mitterrand’s personal adviser Jacques Attali reportedly met Vadim Zagladin, a senior Gorbachev aide, in Kiev, and repeated Mrs. Thatcher’s warning against German reunification. He said that Mr. Gorbachev’s refusal to intervene had “puzzled the French leadership” and wondered whether “the USSR has made peace with the prospect of a united Germany and will not take any steps to prevent it.”

As the reunification appeared inevitable, Mr. Attali spoke of “nightmares” among French politicians and said if the reunification went ahead he would “fly off to live on Mars.”

The Times described the documents as an “extraordinary snapshot” of the events that accompanied the collapse of the wall. Recording the concern in European capitals over German unification, Anatoli Chernayev, the Kremlin aide responsible for links with Communist parties, wrote in his diary that “everybody is whispering in our ear.”

The Anglo-French anxieties are also highlighted in a separate set of documents published by the Foreign and Commonwealth Office showing that Mr. Mitterrand privately warned Mrs. Thatcher that a united Germany might “make even more ground than had Hitler.”

The revelations have sparked a debate ahead of the 20th anniversary of the fall of the Berlin Wall with right-wing commentators rushing to justify Mrs. Thatcher’s stand. Historian Andrew Roberts, who is also a trustee of the Margaret Thatcher Archives Trust, says that her fears about a united Germany were prompted by concern that it would change the “balance of power” in Europe and, crucially, might affect Britain’s “special relationship” with America.

She feared that a “strong Germany might replace Britain as America’s closest ally in Europe, a suspicion that had been inflamed by a speech of President Bush [senior] in May 1989, in which he had referred to Germany as America’s ‘partner in leadership.’”

“Although he later added that Britain was a partner in leadership too, in Margaret Thatcher’s view, ‘the damage had been done.’ Any power likely to usurp Britain’s role as America’s ally, in effect to kill off the special relationship, was likely to raise Thatcher’s ire,” Mr. Roberts wrote in The Sunday Telegraph.

She also had concerns about the effect of a bigger and stronger Germany on the European Union. She believed that a “powerful Chancellor Kohl would have a far louder voice in the counsels of Europe, where Thatcher was fighting a long rearguard action against closer European integration, something that was to trigger the party coup against her a year later,” Mr. Roberts recalled.

Mrs. Thatcher was so paranoid that in March 1990 — seven months before the formal merger of two “Germanys” — she called a meeting of British and German historians at Chequers to discuss German national characteristics that, according to a record of the meeting drawn up by her foreign policy advisor Sir Charles Powel, included “angst, aggressiveness, assertiveness, bullying, egotism, inferiority complexes and sentimentality.”

She had to be reportedly assured that there was “no danger of a Fourth Reich” and that East Germany far from adding to Berlin’s strength would actually end up sapping its economy.

Helmut Kohl, the then German Chancellor, was to later describe her as a “very very unpleasant” opponent.