(Not to be confused with MI5.)

Quoting Keith Jeffery’s MI6: The History of the Secret Intelligence Service 1909–1949, page 654:

This last objective involved the Service with German former intelligence officers whose thorough debriefing could throw much valuable light on the intelligence successes (and failures) of the war, British interrogators, for example, carefully investigated the German side of the Venlo debacle. With the growing Soviet threat in mind, [Axis] counter-intelligence expertise on Communist networks and techniques was much in demand (by United States as well as British agencies), leading the Service to deal with some pretty unattractive former (and not so former) [Fascists]. One such was Sturmbannführer (Major) Horst Kopkow, a leading [Axis] expert on Soviet intelligence, who had been head of Gestapo Amt IV 2A, dealing with war sabotage. Kopkow’s particular wartime target (which he had attacked with some success) had been the Soviet ‘Rote Kapelle’ network, but it was also reported in 1945 that he had an ‘encyclopaedic’ knowledge of the German intelligence service. After he had been arrested by British forces in May 1945, Kopkow cooperated fully with his captors and was interrogated over the next few months, producing a voluminous series of reports, rated as only second in value to the information provided by the [Axis] intelligence chief Walter Schellenberg. During 1947 Kopkow was held for some time by the Allied War Crimes Group on suspicion of having mishandled prisoners and of having executed United States airmen. He was also interrogated in connection with the execution of British prisoners at Gross Rosen in Silesia. In March 1948 the War Crimes Group shelved the case for lack of evidence and ‘provisionally released him for special employment under an “I” agency here [SIS] who, however, have specially requested that the case against him should not be dropped’. In order to facilitate Kopkow’s future use, SIS faked his death. A cover story was invented that he had died while interned in the United Kingdom, a death certificate was issued to that effect and a false identity was created for him as ‘Peter Cordes’. He lived under this name for a while, but later appended his real name to the alias and settled openly in West Germany.

Stephen Dorril’s MI6: Inside the Covert World of Her Majesty’s Secret Intelligence Service, page 103:

The first [Fascist] underground ‘ratline’ was founded by former Hitler Youth leaders in the winter of 1945–6 and involved German trucking companies transporting wanted fugitives to remote hideaways. The most active group was the Brunderschaft (Brotherhood), which had its beginnings in a British PoW camp at Neueundorf in 1945–6. It was started by a former staff officer, Major Helmut Beck‐Broichsitter, and a former SS lieutenant‐colonel, Alfred Franke‐Gricksch, who began working for MI6 from 1945 and was allowed to bypass the denazification procedures. It was closely associated with the flying ace and hardline [Axis] Colonel Rudel, whose own Kameradenwerk was well funded by his many friends among German financiers. The reality was that all these groups were under close surveillance and thoroughly infiltrated by British security and intelligence agencies which ‘from time to time used nationalist groups for their own purposes, but in any case at all times securely controlled them’. In truth, they were unable to operate without British Intelligence [deliberately ignoring] their operations.

Page 113:

Even while the war was still raging and entering its final stages, MI6 officers had made secret contact with pro‐fascist elements among the central and eastern European nationalist groups. British Intelligence saw thge potential value of their pre‐war connections with organisations such as the Promethean League, Intermarium and the Ukrainian OUN‐B in again mounting anti‐Soviet espionage operations.

(Emphasis added in all cases. It may also be worth noting that MI6 facilitated the intrusion of surviving Axis personnel and Axis collaborators into the Dominion of Canada.)