The UK’s intelligence services are divided into two main agencies MI5 (mainly internal security) and MI6 (mainly working abroad), but what happened to MI1, MI2, MI3 and MI4? Do they represent the active intelligence agencies like the Army’s intelligence section? Or have they just become defunct?
As well as the currently extant MI5 and MI6, there have been a number of British military intelligence groups designated as MI-(section number) existing at various times since WWI, which have now been abandoned or subsumed by MI5, MI6 or GCHQ. These included MI1 (codebreaking), MI2 (intelligence in Russia and Scandinavia), MI3 (Eastern Europe), MI4 (aerial photographic interpretation), MI8 (signals intelligence), MI9 (covert operations and PoW escape), MI10 (weapons analysis) and MI19 (PoW debriefing)
source: wikipedia.com
Usenet post (poor cite, but more detail)
Probert Encylopedia Note that there are a few minor differences.