Julius Rosenberg, 33, and his 35-year-old wife, Ethel, were accused of stealing technical information from the atom research centre in Los Alamos and turning it over to the KGB.

A radar expert, Morton Sobell, has also been found guilty of the same charges.

The court heard the Rosenbergs, who have two young sons, were involved in a complicated spy ring, which also included Mrs Rosenberg's brother, David Greenglass, former Soviet vice-consul Arkadi Yakovlev, and Philadelphia chemist, Harry Gold.

Greenglass, a machinist at the Los Alamos research centre during World War II, said he had been asked by the Rosenbergs, both committed Communists and members of the Young Communist League, to obtain information about the atomic bomb.

Greenglass told the court he was unaware he was working on the atomic bomb project until his brother-in-law, Julius Rosenberg, told him.

The court heard the information was passed to Harry Gold, who turned it over to the Russians.

Gold, who is now serving a 30-year jail sentence after pleading guilty to espionage, had also worked as a go-between for British scientist Klaus Fuchs, it was revealed.

Fuchs was jailed for 14 years in 1950 after admitting that he had been passing atomic secrets to the Russians for many years.

Arkadi Yakovlev, also allegedly involved in the spy ring, escaped trial after fleeing to Russia before the American authorities could catch up with him.

In pronouncing guilty verdicts, Judge Kaufman, presiding over the trial, said: "That citizens should lend themselves to the destruction of their own country by the most destructive weapon known is so shocking that I cannot find words to describe the loathsome offence."

The couple, who have consistently denied any involvement in the spy ring, will be sentenced on 5 April.

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