The conflicting attitudes towards Philby between the sister services of British intelligence would expose a cultural fault line that predated this crisis, long outlasted it, and persists today. MI5 and MI6 — the Security Service and the Secret Intelligence Service , broadly equivalent to the FBI and the CIA — overlapped in many respects, but were fundamentally dissimilar in outlook.  MI5 tended to recruit former police officers and soldiers, men  who spoke with regional accents and frequently did not know, or did not care about, the right order to use the cutlery at a formal dinner. They enforced the law, defended the realm, caught spies and prosecuted them. MI6 was more public school and Oxbridge; its accent more refined, its tailoring better. Its agents  and officers frequently broke the laws of other countries in pursuit of secrets, and did so with a certain swagger. MI6 was White’s; MI5 was the Rotary Club. MI6 was upper-middle class (and sometimes aristocratic); MI5 was middle class (and sometimes working class)… Philby’s patronising dismissal of Dick White as “nondescript” precisely reflected MI6’s attitude to its sister service: White, as his biographer puts it,  was “pure trade”, whereas Philby was “establishment”.

Ben Macintyre, “A Spy Among Friends”.