THE LATE Lord Rothschild, scientist, think-tank head, first- class cricketer, bomb-disposal expert and MI5 agent, was a super-spy for the Russians, according to a forthcoming book.

The Fifth Man, by the Australian author Roland Perry, claims to prove that Victor Rothschild stole 'all major UK/US weapons developments in the Second World War', including biological warfare, the atomic bomb and radar.

Specifically, he alleges that Rothschild, not Klaus Fuchs, or, as is generally believed, the civil servant John Cairncross, first alerted Stalin to Allied plans to build an atom bomb using plutonium 235.

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Perry also claims that Rothschild, who died in 1990, was involved 'in so many aspects of spying that he seemed like a super-agent, sabotaging every Western intelligence initiative for 20 years after the war'.

The evidence offered is largely derived from three days of interviews in Moscow with seven retired KGB officers, some identified only by initials.

The most important was Yuri Ivanovitch Modin, controller of the Cambridge spies, and orchestrator of the Burgess/Maclean defection.

But speaking from Moscow late last week, an 'astonished' Modin denied Perry's version comprehensively. Yes, there had been interviews supposedly as the basis of a documentary film about former KGB officers. But no, he had never hinted, nor did he believe, that Rothschild was the fifth man, or any kind of Soviet agent.

'We knew Rothschild was close to (Guy) Burgess and (Anthony) Blunt (later unveiled as the 'fourth man' in the Burgess and Donald Maclean affair).

Because he was in MI5 they learned things from him. This doesn't make him the fifth man, and he wasn't,' he said.

One explanation may be confusion. Perry attributes to the brilliant Rothschild a number of espionage coups which Modin knows from archives and personal contact were the work of the unassuming Cairncross. Modin provides corroborating detail in his own book, My Five Cambridge Friends, to be published by Hodder/Headline on 27 October.

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So grateful were the KGB for Cairncross's early atom secrets, gleaned while secretary to the wartime scientific chief, Lord Hankey, that the Soviet ambassador presented him with the Order of the Red Banner at a secret ceremony in London in 1944. The insignia, in a velvet- lined case, were then taken back to the embassy.

Even Perry does not suggest that 'Victor', as everyone called him, was offered any such dubious honour to add to the British mention in dispatches and US awards he received for dangerous war work. A man who had proved his courage racing cars and batting against the fast bowler Harold Larwood, he became the top expert on bomb disposal. He was also responsible for the personal security of his friend Winston Churchill, testing, and sometimes tasting, the Prime Minister's gifts of exotic food and drink.

Rothschild survived the war and the Burgess/Maclean defection to become the acknowledged 'grand old man of espionage' consulted by successive governments. However, rumours based on his close association with Burgess, Maclean and Kim Philby, all of whom were habitues of his wartime house, surfaced again in the 1980s when he became embroiled in the Peter Wright Spycatcher affair.

In 1986 Rothschild wrote a public letter avowing his innocence, and soon after Mrs Thatcher issued a famously terse comment that 'we have no evidence he was ever a Soviet agent'.

As a clearance it was less than fulsome, but when he died in 1990 the Prime Minister attended his memorial.

A clue to her coolness emerged from the interviews of Kim Philby by the writer Phillip Knightley in Moscow. Philby believed that leaving MI5 in 1947, Rothschild had seized or copied all the six-by-four file- cards listing Soviet agents in Europe and elsewhere. These, Philby believed, were used by Mossad, the security service of the fledgling state of Israel, whose cause Rothschild's family had championed from the 19th century.

Mrs Thatcher may have taken the view that his conflict of loyalties was not between Whitehall and the Lubyanka, but his country of birth and his family's Zionist dreams. In his lifetime inquiries, let alone speculation about his exotic career as scientist and spymaster, were inhibited by his lawyer, the redoubtable Lord Goodman. But as William Armstrong of Sidgwick & Jackson, the book's publisher, said, 'dead men can't sue for libel'.

Rothschild's ghost and his surviving family may well wish that death did not inhibit libel actions, all the more so as 'Rothschild family members' are cited in the book describing his low spirits in the years before his death.

But their lawyers, Freshfields, said that in the circumstances their reaction would be 'a dignified silence'.

(Photograph omitted)