Circling the Lion's Den


Structure of MI5

Senior Management

  • Director-general
  • Deputy Director-general
  • Director and Co-ordinator of Intelligence (responsible for Northern Ireland)
  • Legal Adviser


A Branch - Operational Support

  • A1A: Technical operations, covert entry, eavesdropping devices, CCTV coverage of premises
  • A1F: As above, but on longer-term targets like embassies and head
  • A2A: Transcription of intercept material
  • A3 and A5: Technical support for operations. This includes specialised covert phtotography and lockpickers to assist in covert entries.
  • A4: Mobile and static surveillance units

B Branch - Human Resources

  • B1: Protective security for MI5, including security of building and vetting of MI5 staff
  • B2: Personnel
  • B7: Training and recruitment

D Branch - Non-Terrorist Organisations

  • D1: Vetting of people outside MI5
  • D4: Counter-espionage, notably on Russia and China
  • D5: Agent runners for this branch
  • D7: Organised crime

G Branch - International Terrorism

  • G2P: Counter-proliferation
  • G3A: Co-ordination of threat assessments
  • G3C: Countering terrorism in other parts of the world not covered elsewhere by G Branch
  • G6: Agent running for G Branch
  • G9A: Countering threats from Libya, Iraq, Palestinian and Kurdish groups
  • G9B: Countering threats from Iranian state terrorism and Iranian dissident groups
  • G9C: Countering Islamic extremists

H Branch - Corporate Affairs

  • H1 and H2: Liaison with Whithall and the media. Covert financial enquiries with financial institutions. This section liaises with the police, policy, custom, ports and immigration. It’s also responsible for management policy, including information technology.
  • H4: Finance
  • The following sections are also part of H Branch
  • R2: Main registry
  • R5: Y-boxed files. These are files with restricted access within the Service.
  • R10: Registry for temporary files
  • R20: Responsible for administering GCHQ material

T Branch - Irish Terrorism

  • T2A: Investigates Republican and Loyalist terrorism on the British mainland
  • T2B: Liaises with local Special Branches and agent runners
  • T2C: Assesses threats from Irish terrorist groups
  • T2D: Researches Irish terrorist groups
  • T2E: Liaises with Metropolitan Police Special Branch, based at Scotland Yard
  • T5B: Investigates arms trafficking
  • T5C: Counters Irish terrorism in continental Europe, including the Republic of Ireland
  • T5D: Counters Irish terrorism in the rest of the world
  • T5E: Studies terrorist logistics
  • T8: Runs agent for T Branch; includes a section based in Northern Ireland

PS. MI5 also has security and defence liaison officers based in Germany,WashingtonDC and Ciprus.


Defending the ralm / Inside MI5 and The War On Terrosism. Mark Hollingsworth and Nick Fielding