A private account has emerged of a trip in 1941 to Copenhagen in occupied Denmark by the German nuclear physicist Werner Heisenberg during which he alerted his former mentor, Niels Bohr, to Hitler’s “uranium club”.

According to experts who have seen them, the letter and other documents confirm for the first time that Heisenberg leaked the fact that Hitler was trying to build an atomic bomb. They are, however, not explicit on his motives for doing so.

He may have been trying to get a warning to the allies about Hitler’s plans but — as seems more likely — he may have leaked the information in a botched attempt to “turn” the Danish scientist or extract information from him.

Either way, Bohr reworked his own calculations and two years later fled to America where he took part in the Manhattan project, which produced the atom bombs that were dropped on Japan and ended the seond world war in 1945.

The encounter between Heisenberg and Bohr inspired the West End and Broadway play Copenhagen, in which the British playwright Michael Frayn speculated on its significance to the Nazis’ eventual failure to develop an atomic bomb.

The new evidence consists of an unposted letter from Bohr to Heisenberg, written in 1958, and 10 other documents written before his death in 1962. It reveals that Heisenberg alerted Bohr to the weapons programme.

“Essentially, the letter shows that he told Bohr that it was possible that the war would be won with atomic weapons, indicating that he was involved in such work,” said Dr Finn Aaserud, the director of the Niels Bohr archive in Denmark, which will release the papers on February 5.

What remains a mystery is Heisenberg’s motive, which Aaserud says is not made clear in the papers. However, it is known that the two men — whose collaboration in the 1920s and 1930s produced many early breakthroughs in nuclear technology — fell out after the war when Heisenberg published his account of the Nazi nuclear programme in 1958, suggesting that he had always intended to sabotage Hitler’s nuclear ambitions.

Bohr’s unposted letter was written in response to Heisenberg’s apologia. According to Gerald Holton, a Harvard physics professor who is the only living man outside Bohr’s family and archive team to have studied the letter, Bohr decided not to send it at the time because he shrank from confrontation. “When he re-read the letter he thought that it was uncharacteristically strong. Therefore he put it to one side but did not destroy it.”

Heisenberg, although initially hounded by the Nazis for practising the “Jewish” physics of Einstein, was put in charge of the Nazi bomb programme. In September 1941 — with most of Europe subdued, Hitler’s armies at high tide having almost reached Moscow and the war apparently won — Heisenberg took a train to Copenhagen. He went for dinner at Bohr’s house, after which the two men took a walk outside, perhaps to escape any hidden microphones. It is thought that Bohr was so disturbed by what Heisenberg said during this walk that he abruptly ended the conversation.

What happened during this walk has been the subject of so much speculation, particularly after Frayn’s play, that Bohr’s family decided to bring forward by 10 years the release date of the documents.

Whatever Heisenberg’s motives, Aaserud said, it was “vastly controversial” and “very dangerous” of him to confide the extent of the German atomic programme.

In his own account of his time working for the Nazis, Heisenberg hinted that he had been aware of the the importance of depriving Hitler of the bomb even while working on the programme.

The letter is Bohr’s reply to this defence. After his death, it was found folded inside a copy of the book in which Heisenberg’s apologia was published.

Holton described the letter as “astounding” and said its release would help to solve one of the most intriguing episodes of the second world war.

“It contains the missing part of the whole story,” he said.

Additional reporting: Peter Day, Rachel Dobson