They had been convicted of conspiring to steal US atomic secrets for the Soviet Union. The prosecution's case rested mainly on the testimony of Mr Greenglass.

At the trial in 1951, he said his sister typed up notes containing US nuclear secrets that were later turned over to the KGB, the Soviet intelligence service.

The notes - typed on a portable Remington typewriter - apparently contained little that was new to the Soviets, but for the prosecution they clinched the case against Ethel Rosenberg.

No recollection

However, in this week's television interview, Mr Greenglass said his testimony was not based on first-hand knowledge:

"I don't know who typed it, frankly, and to this day I can't remember that the typing took place. I had no memory of that at all - none whatsoever."

He said he gave false testimony to protect himself and his wife, Ruth, and that he was encouraged by the prosecution to do so.

"I would not sacrifice my wife and my children for my sister."

Mr Greenglass, who worked on the atomic bomb at the top-secret Manhattan Project in Los Alamos during the Second World War, was himself convicted of giving the Soviets information about nuclear research - but was spared execution in exchange for his testimony.

He spent 10 years in prison and was released in 1960. Today, he and his wife live under an assumed name in the New York area.

Asked whether his false testimony against his sister still haunted him 50 years later, Mr Greenglass said: "Every time I am haunted by it, my wife says 'Look, we are still alive.'"

Anti-communist hunt

Ethel and Julius Rosenberg were the only people in the United States ever executed for Cold War espionage, and their conviction fuelled US Senator Joseph McCarthy's anti-communist crusade against "anti-American activities" by US citizens.

While their devotion to the communist cause was well documented, they denied the spying charges even as they faced the electric chair.

Their defenders said they never stood the chance of a fair trial given the anti-communist sentiment in the US in the 1950s.

No apology

But others saw the Rosenberg case as proof of a communist conspiracy.

Some 50 years later, Mr Greenglass believes he will be remembered by history as "a spy that turned his family in". But, he says, he does not care.

"I had no idea they would give them the death sentence."

And he says if he ever met the Rosenbergs' two sons he would say "sorry your parents are dead" but he would not apologise for his actions.