The 88 year-old author, Luigi Romersa, is the last known witness to what he and some historians believe was the experimental detonation of a rudimentary weapon on an island in the Baltic in 1944.

Hitler's nuclear programme has become a subject of intense dispute in recent months, particularly in Germany. An independent historian, Rainer Karlsch, met with a barrage of hostility when he published a study containing evidence that the Nazis had got much further than previously believed.

Mr Romersa, a supporter of Mr Karlsch's thesis, lives today in an elegant flat in the Parioli district of Rome. His study walls are covered with photographs from a career during which he interviewed many of the major figures of the 20th century, from Chiang Kai-shek to Lyndon Johnson. Though he suffers from some ill-health these days, he is still lucid and articulate.

He told the Guardian how, in September 1944, Italy's wartime dictator, Benito Mussolini, had summoned him to the town of Salo to entrust him with a special mission. Mussolini was then leader of the Nazi-installed government of northern Italy and Mr Romersa was a 27 year-old war correspondent for Corriere della Sera.

Mr Romersa said that when Mussolini had met Hitler earlier in the conflict, the Nazi dictator had alluded to Germany's development of weapons capable of reversing the course of the war. "Mussolini said to me: 'I want to know more about these weapons. I asked Hitler but he was unforthcoming'."

Mussolini provided him with letters of introduction to both Josef Goebbels, the Nazi propaganda chief, and Hitler himself. After meeting both men in Germany, he was shown around the Nazis' top-secret weapons plant at Peenemünde and then, on the morning of October 12 1944, taken to what is now the holiday island of Rügen, just off the German coast, where he watched the detonation of what his hosts called a "disintegration bomb".

"They took me to a concrete bunker with an aperture of exceptionally thick glass. At a certain moment, the news came through that detonation was imminent," he said. "There was a slight tremor in the bunker; a sudden, blinding flash, and then a thick cloud of smoke. It took the shape of a column and then that of a big flower.

"The officials there told me we had to remain in the bunker for several hours because of the effects of the bomb. When we eventually left, they made us put on a sort of coat and trousers which seemed to me to be made of asbestos and we went to the scene of the explosion, which was about one and a half kilometres away.

"The effects were tragic. The trees around had been turned to carbon. No leaves. Nothing alive. There were some animals - sheep - in the area and they too had been burnt to cinders."

On his return to Italy, Mr Romersa briefed Mussolini on his visit. In the 1950s, he published a fuller account of his experiences in the magazine Oggi. But, he said, "everyone said I was mad".

By then, it was universally accepted that Hitler's scientists had been years away from testing a nuclear device. Allied interrogators who questioned the German researchers concluded that there were vast gaps in their understanding of nuclear fission. In any case, the US had needed 125,000 people to develop the atomic bombs dropped on Hiroshima and Nagasaki in 1945, whereas Germany's programme involved no more than a few dozen physicists, led by the Nobel laureate Werner Heisenberg.

But documents published recently by Mr Karlsch and an American scholar, Mark Walker of Union College, Schenectady, have punctured this consensus. Russian archives have shown that one of the German scientists lodged a patent claim for a plutonium bomb as early as 1941 and, in June, the two historians published an article in the British monthly, Physics World, that included what they claimed was the first diagram of one of the bombs Hitler's scientists were trying to build - a device that exploited both fission and fusion.

The true novelty of Mr Karlsch's research, though, is to have turned the spotlight off Heisenberg and onto a competing project run by one Kurt Diebner. A Nazi since 1939, Diebner had his own group at Gottow near Berlin. Mr Karlsch found evidence to show that, sponsored by Walther Gerlach of the Reich Research Council, this group abandoned its quest for an A-bomb to concentrate on a weapon made of conventional high explosives packed around a nuclear core. "It was a tactical battlefield weapon they probably wanted to use against the approaching Soviet armies," said Professor Walker.

Could Mr Romersa have seen the detonation of an early prototype? He is not the only person to have claimed to have witnessed similar explosions. Former East German archives have produced this account by Cläre Werner: on the evening of March 3 1945, she claimed, she was near the town of Ohrdruf when she saw a "big, slim column" rise into the air, "so bright that one could have read a newspaper".

Ohrdruf had a concentration camp, part of the Buchenwald complex. Heinz Wachsmut, who worked for a local excavating company, told officials that the day after Ms Werner claimed to have seen an explosion he was ordered to help the SS build wooden platforms for the cremation of the corpses of prisoners. He said their bodies were covered with horrific burns.

After the war, the scientists engaged in the Nazi project were interned. Gerlach, whose research in other fields won him praise from the likes of Albert Einstein, returned to academic life and died a revered figure. Diebner eventually got a job in West Germany's defence ministry. Neither man ever alluded to their work on what would have been the world's first tactical nuclear weapon.

"Diebner and Gerlach said nothing about this," said Prof Walker. "They took it to their graves."

· Le armi segrete di Hitler, by Luigi Romersa, is published by Ugo Mursia Editore. €14