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Confessions of an atom spy: Forty years after Bruno Pontecorvo, a British scientist, went to work for Moscow, he tells Charles Richards in Rome why he changed sides

He stepped out of the lift on the third floor, entered the room, and smiled. Now 78 and a victim of Parkinson's disease, Bruno Pontecorvo is short and spare, with an old tweed jacket hanging off his drooping shoulders. His pale green eyes, however, still gleam with mischief.

He spends most of his time in Italy these days, but for a year he had fobbed me off, saying over the phone that he was too sick and did not give interviews about the past. Now at last we were meeting at the home of Miriam Mafai, a columnist on the daily La Repubblica who has just published a book about him.

A physicist, Pontecorvo was deeply involved in the British nuclear research programme when, in 1950, he decided during a holiday in Italy that he would abandon the West and make a new life in the Soviet Union. His defection, following closely on the unmasking of the atom spy Klaus Fuchs, a colleague at the government atomic research centre at Harwell, deeply embarrassed the Attlee government. Had Pontecorvo taken with him more atomic bomb secrets? Had he been spying for the Russians all along? He was vilified for treachery and stripped of his adopted British nationality.

Now, for the first time, he is prepared to talk about the choice he made. But, with most Communist countries having changed their colours, how does he feel about the dedication of his life to the Communist cause?

'The simple explanation is this: I was a cretin,' he said. 'The fact that I could be so stupid, and many people close to me should have been quite so stupid . . .' The sentence was left unfinished.

Communism, he went on, was 'like a religion, a revealed religion . . . with myths or rites to explain it. It was the absolute absence of logic.' He stuck by his faith, even after the invasion of Hungary in 1956. When Andrei Sakharov, a fellow physicist, turned against the system, it made no difference. 'I had always admired him as a great scientist and a man of integrity. However, my idea was that he was naive . . . it was I who was naive.'

It was only after Czechoslovakia that his views began to change. 'After 1968 I would say, I will not talk of such things. Then, after a few years, I understood what an idiot I was.'

He speaks clear, idiomatic English. He is a man of immense charm and elegance of language, quoting from Dante, which he read in the Soviet Union to remind himself of Italy.

Born in Pisa in 1913, Pontecorvo was the fourth of eight children of a textile merchant. He showed early academic promise, and in 1934 went to Rome to join an extraordinary circle of world- class physicists led by Enrico Fermi, later to become one of the fathers of the atomic bomb. When Mussolini passed racial laws that discriminated against Jews, four of the Pontecorvos went to Britain. (The eldest, Guido, a distinguished geneticist, remained in Britain and is a Fellow of the Royal Society.)

Bruno went to France and then, in 1940, to the United States. Three years later he was asked to join an Anglo-Canadian team conducting secret research at Montreal. This was Britain's wartime nuclear reactor programme, which was separate from the American-led Manhattan Project working on the first atomic bombs. In 1948, while still working in Canada, Pontecorvo became a British subject. In January 1949 he left with his family to take up a senior post in the Atomic Energy Research Establishment at Harwell, near Oxford. It was not a nuclear weapons plant, although such was the secrecy surrounding its work that it was popularly believed to be.

In the late summer of 1950, Pontecorvo went on holiday to Italy with his family, and disappeared. It was widely assumed that the Pontecorvos flew to Stockholm, spent the night in a building belonging to the Soviet Union, and flew on to Helsinki. Then a Soviet vessel left the city's harbour shortly afterwards.

Two weeks passed before the alarm was raised in Britain, and another sensational security scandal broke over the government's head. How much did Pontecorvo know? Had he been vetted? Had he been in league with Fuchs? As always with these matters, the principal casualty was Britain's standing in Washington, where once again it was exposed as a soft touch for Soviet subversion. Very soon the relevant minister was forced to admit: 'I have no doubt that he is in Russia.'

It was not until 1955 that Pontecorvo surfaced in Moscow, giving a press conference at which he said he had defected to correct the balance between East and West and that he had only ever worked on the peaceful uses of atomic energy. 'I became convinced,' he said, 'that in the Soviet Union the people wanted peace, and that their government was doing everything possible that there should be no war.'

What had prompted him to defect at that moment, who had helped him through the Iron Curtain and how exactly he made that journey, remained a mystery. Now, the first new evidence is emerging. A week ago, an old Italian Communist Party hardliner, Giulio Seniga, said that he had been a member of an underground party network that arranged Pontecorvo's flight to Moscow. And Pontecorvo himself revealed to Miriam Mafai how he crossed the Soviet border. 'We did not go by ship,' he said. 'We went straight to the Soviet embassy, and then we left in two cars, with me locked in the boot.'

But had he spied for Moscow before then? He still does not talk about it. Fuchs himself told the British that he believed someone else, probably at Harwell, was talking to the Russians, and spy writers Christopher Andrew and Oleg Gordievsky are in no doubt that it was Pontecorvo. 'KGB officers,' they have written, 'said that they rated Pontecorvo's work as an atom spy almost as highly as that of Fuchs.'

In 1950, certainly, the press was quick to denounce him, but today the available evidence does not appear to support the suggestion that he was an important atom spy. His research at Harwell had no military application and in Canada he had worked on reactors rather than weapons.

At any rate, Pontecorvo today is not a wanted man. He has been visiting Italy regularly since 1978, although his home remains in Dubna, the research centre outside Moscow where he has been living with his wife and children. He has, he says, a lingering nostalgia for the country whose citizenship he held for a dozen years in the middle of his life, and to which he insists he did no harm. 'I love England, the cathedrals. It is also the least corrupt country.'

For a verdict on the life of Bruno Pontecorvo, there may be no better judge than his brother Gillo, the director who won international acclaim for his film, The Battle of Algiers. 'The case of Bruno was very simple,' he told me. 'Forget all the lies told about it. We were living at a period of great change. There was this belief in a city of the future. The belief was almost irrational, but was held by a whole generation of men, above all intellectuals . . . He had that religion, that sense of capitalism equalling war and recurrent crisis and racism. All that had to be overthrown to go towards the new world. They were like the early Christians, who believed in something beautiful, which did not exist.

'We bet on something which turned out to be false. It is like stepping out of a window, and hoping to descend slowly without taking the boring route of the stairs or the lift. We ignored the problem of the law of gravity.'

(Photograph omitted)