Extracts from Kim Philby’s official confession to the UK’s security services in which he likens joining the Soviet secret police to signing up to the army, have been made public for the first time.

Philby, one of the Soviet Union’s most notorious British cold war spies, fled to Moscow shortly after his 1963 admission of guilt.

His confession, dated 11 January 1963, headed “secret”, has been published partially in the latest tranche of MI5 files to be released to the National Archives.

It includes Philby, who, while a double agent at one time headed up counter-espionage operations for MI6, explaining that joining the Soviet secret police (OGPU) did not mean he agreed unquestioningly with everything they did.

“None of the OGPU officials with whom I had dealings ever attempted to win my total acceptance on the technical level,” he said.

“In short I joined the OGPU as one joined the army. There must have been many British soldiers who obeyed orders at Passchendaele [Britain’s most controversial first world war battle] although convinced they were wrongly conceived.”

Philby gave the confession to his friend, MI6 intelligence officer Nicholas Elliott in Beirut, almost 30 years after his recruitment in Regent’s Park in 1934. MI5 wanted to send its own interrogator to confront Philby but MI6 believed the double agent would be more likely to open up to an old colleague. The extracts, published on Tuesday, begin with the rendezvous in the London park, with the man who would become Philby’s handler, Arnold Deutsch, known to the Briton only as “Otto”.

Philby was the son of a British empire official and privately educated before attending Cambridge University. He said Deutsch told him that “a person with my family background and sensibilities could do far more for communism than the run-of-the-mill party member or sympathiser”.

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Philby was the most successful of the British communists who spied for Moscow, known as the Cambridge Five. He revealed in the confession that one of his first tasks was to identify fellow Cambridge communists and after he presented a list, Deutsch and another Soviet spy, introduced to him as “Big Bill”, settled on Guy Burgess and Donald Maclean as potential recruits.

Philby said: “I was in favour of recruiting Donald, but entered strong reservations with regard to Guy, on the grounds of his unreliability and indiscretion.” However, he went on to say that he was overruled and Burgess “made no more difficulty” than Maclean.

In 1951, both would defect to the Soviet Union, after a tip-off from Philby that they were under suspicion.

The confession extracts end with Philby’s account of the last time he saw “Otto”, when his handler, who would be replaced by “Theo”, discarded his normal security precautions to call him at home and inform him he was coming round.

“He arrived in a state of great agitation with a suitcase,” said Philby. “He used my telephone to book an air passage to Paris, and left the following morning. I never saw him again.”

Handwritten notes on the confession say this incident took place in 1936, although elsewhere in the files Deutsch is recorded as leaving the country the following year. Philby was under suspicion from 1951 but was protected by a mixture of incompetence and incredulity that he could be guilty.

In 1963, Elliott confronted Philby, who was by then working as a journalist in Lebanon and had been officially cleared of spying in 1955, to offer him immunity from prosecution if he returned to London and provided a full confession. Instead, Philby stalled, providing small pieces of information before defecting onboard a Soviet freighter on 23 January 1963.

Given the huge embarrassment MI6 and the Foreign Office would have sustained if it became known Philby was a spy, some believe he may have been allowed or even encouraged to flee.

By contrast, for Deutsch, the recruitment of Philby and the other members of the Magnificent Five (Burgess, Maclean, Anthony Blunt and John Cairncross) was his most celebrated achievement.

The files reveal that Philby’s handler tried to use the connection of his cousin, Oscar Deutsch, the millionaire owner of the Odeon cinema chain, as cover for his work for Soviet intelligence work, while in the UK.

His cousin, probably unaware of his intelligence role, told the Home Office in 1936 that he wanted to employ Arnold Deutsch at a salary of £250 as an “industrial psychologist”, noting that he had “made an intensive study of psychology in relation to the cinema”.

The post was described as investigating among other things the “popularity of certain types of films”. But the file notes that Arnold Deutsch’s application was rejected on the basis that a qualified British psychologist could easily be found, “therefore it was not considered that the employment of the alien should be recommended”.