By Paul Reynolds BBC News Online

A stinging attack on the police was made by the head of MI5 when a proposal was made in 1945 to hand over responsibility for the surveillance of Communists and "subversives" to Scotland Yard's special branch, a letter released by the Public Record Office has revealed.

In a letter to the Home Office from Box 500, Parliament Street (MI5's cover address), the head of the service, Sir David Petrie, was scathing in his comments about the police, saying that they were not up to the job and might be corrupt.

Petrie said: "It is my considered opinion that the police by themselves are not competent to take over what MI5 now does."

The Home Office, he said, would be making "a grievous mistake if ever they divorce MI5 from its present activities".

The lower ranks... cannot always be expected to appreciate the need of complete secrecy

Sir David Petrie

As a former policeman himself, he stated, he was not pre-disposed to be a "detractor of men of my own totem".

But he went on, in a comment which seems loaded with class and professional prejudice: "You want... officers of a different calibre from the generality of police officers in this country, not excepting the higher ranks.

"You need, in fact, people with much the same educational background and the same mental equipment as you employ in the higher grades of the Civil Service."

He comments that the police are "heavily loaded down with their ordinary duties" unlike the "centrally situated Hollis" - a reference to Roger Hollis who himself came under suspicion at one stage in the spy scandals of the 50s.

Petrie clearly had no doubts about him.

Petrie also claims that the police would not know how to keep a secret, especially "the lower ranks who cannot always be expected to appreciate the need of complete secrecy in the same way as our staff do".

He added, without explanation beyond a reference to some "material shown to you in the summer of 1943", some darker thoughts.

"There is also in the background the ugly question of corruption."

Communist threat

MI5 managed to prevent the police from taking over, despite sympathy for the idea in the Home Office.

It also saw off another proposal for it to be merged with the Secret Intelligence Service MI6, and to be confined to duties like checking ports and war factories (the war had not ended at this time).

The letter also shows how MI5 had already switched its priorities from chasing Nazi spies to seeking Communist ones.

Nobody will ever be able to say, though, whether the police might not have done a better job of stopping the rash of spying cases which developed soon afterwards.