France today revealed secret archives dating from the time of the fall of the Berlin Wall that describe Baroness Thatcher as an "obsessed" and "bitter" leader, deeply opposed the reunification of Germany.

The then British prime minister warned France’s ambassador months before West and East Germany did reunify of a domineering Chancellor Helmut Kohl who “sees himself as the master”, diplomatic notes revealed.

The Elysee Palace unsealed the secret archives to commemorate the 20th anniversary of the Wall coming down on November 9 1989. Confidential briefing notes written by French diplomats in London describe Lady Thatcher’s fear of a united Germany - and her proposal to join forces with Russia to contain the threat.

“Kohl is capable of anything,” Luc de La Barre de Nanteuil, France’s ambassador to Britain, quoted Thatcher as telling him during a dinner with French businessmen at his London residence on March 13, 1990.

“He has become a different man, he does not know himself any more, he sees himself as the master and begins to act like that. You have to see for example how he behaves with (then Soviet leader Mikhail) Gorbachev,” Lady Thatcher reportedly went on.

Nanteuil was struck by the British premier’s bitterness, noting that while she was pleased with the end of communism, she did not express any joy over eastern Europe’s new-found freedom.

“The 1990s begin with euphoria, they risk ending in catastrophe,” said Lady Thatcher, whose anti-European Union stance was later to be instrumental in her forced resignation in December 1990.

France and Britain, western Europe’s two nuclear powers, needed to link up in the face of the “German danger”, but this alone would not be enough, she told Mr Nanteuil. Once Russia was transformed into a free-market democracy, it could act as a necessary counterweight, she said.

While Lady Thatcher’s opposition to German reunification is known, the diplomatic notes cast French President Francois Mitterrand in a more ambivalent light than has previously been thought.

Mr Mitterrand was widely known to have shared Lady Thatcher’s concerns - he has previously been cited as warning her in January 1990 that the Germans were turning “bad” again.

But the diplomatic records also describe his delicate balancing act, as he tried to show Germany he was not fundamentally hostile to its reunification.

At a meeting on November 14 1989, five days after the Berlin Wall came down, French diplomat Francois Scheer told the East German ambassador that the thought of German reunification “did not frighten (Mitterrand) but this did not signify that he would say yes to reunification tomorrow and in any fashion”, according to a report of the encounter.

Archivists presented the notes and letters to reporters with a word of caution, pointing out that diplomats may have been biased while leaders could have tried to manipulate their counterparts.

The full archives from the period will be officially opened on Monday, the 20th anniversary of the fall of the wall.

But even as a partial account, Nanteuil’s report confirmed Lady Thatcher’s increasing isolation as the Iron Lady lost the power to convince others.

“Mrs Thatcher was, as usual, attentive, reflective and combative, but she was basically much more negative than imaginative. She spoke of Germany with a striking anxiety and passion,” Nanteuil wrote.

“She wanted to be right and be right on all points, even when her worry did not suggest any solution. She was, as always, convinced that her instinct was right.”

Times Recommends

• Times Charity Appeal: SolarAid
• Is it Black Monday all over again for Dubai?
• Behind the scenes at the biggest deal