What Do We Know About Britain's Spy Agencies?

Britain's spy agencies gained popularity through the 'James Bond' series. But how much do we know about MI5 and MI6?

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MI5 (mi5.gov.uk)

MI6 (sis.gov.uk)

UK spy agencies have collected bulk personal data since 1990s, files show (guardian.com)
"Disclosure of internal MI5, MI6 and GCHQ documents reveals the agencies' growing reliance on amassing data as a prime source of intelligence even as they concede that such "intrusive" practices can invade the privacy of individuals."

The time when spy agencies didn't officially exist (bbc.com)
"This week the new heads of intelligence agencies MI6 and GCHQ took up their posts."