The conflicting attitudes toward 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 CIA -- overlapped in many respects but were
fundamentally dissimilar in outlook. MI5 tended to recruit former
policemen and soldiers, men who sometimes spoke with regional accents
and frequently did not know, or care about, the right order to use the
cutlery at a formal dinner. They enforced the law and 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
Club; MI5 was the Rotary Club; M16 was upper-middle class (and
sometimes aristocratic); MI5 was middle class (and sometimes working
class). In the minute gradations of social stratification that meant
so much in Britain, MI5 was "below the salt," a little common, and MI6
was gentlemanly, elitist, and old school tie. MI5 were hunters; MI6
were gatherers.... MI5 looked up at MI6 with resentment; MI6 looked
down with a small but ill-hidden sneer. The looming battle over Philby
was yet another skirmish in Britain's never-ending, hard-fought, and
entirely ludicrous class war.
-- Ben Macintyre.
A Spy Among Friends: Kim Philby and the Great Betrayal (2014)
Broadway Books; Reprint edition, 2015, p162.