Blunders that allowed Soviet spy Klaus Fuchs to give Moscow the secrets of the atomic bomb have been revealed in documents released from the security service MI5.
Time after time, the files made public by the National Archives (formerly the Public Record Office) show that Fuchs was not properly investigated, despite information from Germany that he had been a Communist Party member.
Klaus Fuchs was jailed for his spying
One of the MI5 officers supporting Fuchs most strongly was Roger Hollis, who later came under suspicion himself.
Hollis said of Fuchs: "I myself can see nothing which persuades me that Fuchs is in any way likely to be engaged in espionage activity."
But Hollis was not alone in that assessment. His view was part of a pattern.
Fuchs came to Britain as a communist refugee from the Nazis in 1933. A brilliant physicist, he worked on the Manhattan Project at Los Alamos which built the first atomic bomb. In 1950 he confessed that he had spied for the Soviet Union and was sentenced to 14 years in prison.
Of course, it is easy to argue with hindsight that greater checks should have been made, but what emerges from these files is that the police and security services were using the wrong test for Fuchs.
On at least four occasions, questions were raised about him, but the only real issue tackled was whether he was politically active in Britain.
When the answer came back that there was no evidence that he was, he was assumed to be clean. The idea that he might be active as a spy did occur to a few but they were not believed.
It was in 1934 that the German consul in Bristol revealed to the chief constable there that Fuchs had been a communist.
It seems unlikely that he is in close touch with anyone of interest
Security officer Daphne Bosanquet
The information came from the police in Germany: "Fuchs joined the communist party and worked for the party as an orator in the election campaigns. Fuchs was leader of the Sea District, the task of which was to break up the National Socialist Party."
Despite a claim by Prime Minister Clement Atlee, made after the Fuchs trial, that this tip-off had been "looked into" and that "there was no support for it whatsoever", there is no evidence whatsoever in the files that it was ever examined.
The source of the accusation was the Gestapo and therefore not taken seriously. It was the German consulate which was criticised, not Fuchs.
"Their report was possibly biassed (sic)", said the chief constable of Birmingham in one letter.
Typical of the kind of approach taken came in 1941. Fuchs was being considered for secret nuclear work at Birmingham University and MI5 wrote to the chief constable asking if Fuchs had "come to notice on account of his political activities or associates?"
Chief Constable Moriaty replied that he had "not come adversely under notice" and that his professor Rudolf Peierls, himself a suspect at one time, had vouchsafed for his "trustworthiness".
Fuchs had given an assurance, the chief constable said, that he took only a "passive" interest in politics, as he was conscious of the hospitality given to him by Britain. This was taken at face value. It was a more trusting time and there was considerable sympathy for refugees from Nazi Germany, communist or not.
One of the British security officers, Daphne Bosanquet, reported on the basis of an informant that Fuchs "takes part in the usual propagandist activities, but is never prominent in them. He bears a good personal reputation and is considered a decent fellow".
Fuchs was trusted at the very highest level
By that time, January 1943, Fuchs had been in contact with the Soviet Embassy in London for more than a year.
Ms Bosanquet said later in 1943: "As he has been in his present job for some years without apparently causing any trouble, I think we can safely let him continue in it."
She also said: "It seems unlikely that he is in close touch with anyone of interest."
This assessment was made simply on the basis of a two week check on his mail.
There were some doubters but they were swept aside.
One MI5 officer named A. d'Arcy Hughes challenged Daphne Bosanquet by saying: "Surely the point is whether a man of this nature who has been described as being clever and dangerous should be in a position where he has access to information of the highest degree of secrecy.
"He would not likely to cause trouble but might well be expected to be passing information to undesirable quarters."
That was spot on. But it made no difference and Fuchs was cleared to go to America to work on the atom bomb.
The papers released show gaps in MI5 security
Major Garrett of MI5 wrote: "He is rather safer in America. It would not be easy for Fuchs to make contacts with communists there."
He did not have to make contact with communists. He got in contact with Soviet officials in New York and duly passed on the secrets of the atom bomb to his Soviet contact, Harry Gold.
The crucial inconsistency in the MI5 approach was queried by another officer, Michael Serpell, in 1946: "The quietness of Fuchs' political behaviour here does not in the least alleviate the danger to security."
Serpell, too, was ignored.
The files also reveal a British decision not to tell the Americans too much about Fuchs' background while he was at Los Alamos.
Major Garrett stated in 1944: "It would not appear desirable to mention his proclivities to the authorities in the United States."
On return from Los Alamos, Fuchs worked at the nuclear research establishment at Harwell.
After being exposed by information from decoded Soviet intercepts, he confessed in 1950 (first to his mistress in a hotel in Richmond) and a week later to MI5 interrogator Jim Skardon.
Fuchs later went to live in East Germany. MI5 agent Guy Liddell summed him up: "His motive was ideological; his recruitment self-sought."