Cambridge spy walked free because brother was official adviser

● Communist secret agent made partial confession to MI5 after being forced out

A Communist spy who spent years passing secrets to the Russians escaped prosecution because his brother was a senior government adviser, according to newly-released official files.

John Cairncross was one of the notorious Cambridge Spies - along with Kim Philby and Guy Burgess - recruited by Soviet intelligence while at university in the 1930s.

He went on to hold a series of sensitive government posts and is thought to have been the first agent to alert the Soviets to Britain’s programme to develop an atomic bomb, prompting Stalin to launch his own nuclear programme.

In 1964 - having been forced out of government service over suspicions of espionage in 1951 - he finally made a partial confession to MI5.

His admission raised the question as to whether he would be prosecuted.

The files show there were concerns over whether he could be extradited from the US where he was now working as a university lecturer.

In a note to prime minister Sir Alec Douglas-home, cabinet secretary Sir Burke Trend also raised the delicate issue of his brother, Alec Cairncross, who happened to be the government’s chief economic adviser.

He wrote: “Quite apart from the distress and embarrassment which this would inflict