Scotland Yard has arrested a 37-year-old man on suspicion on manufacturing improvised explosive devices in Iraq in 2007.
Officers from the Metropolitan Police's Counter Terrorism Command detained the man under Section 41 of the Terrorism Act 2000 after swooping at his address in northwest London.
A statement said: "The man has been arrested on suspicion of being concerned in the manufacture of improvised explosive devices (IEDs) in Iraq in 2007."
Officers are searching the address while the suspect has been taken to a south London police station where he remains in custody.
IEDs accounted for nearly two thirds of all deaths suffered by American and British forces and their allies during the 2003-2011 Iraq war.
A device placed or fabricated in an improvised manner incorporating destructive, lethal, noxious, pyrotechnic or incendiary chemicals and designed to destroy, incapacitate, harass or distract. It may incorporate military stores, but is normally devised from non-military components.