MI5 was an intelligence body of the United Kingdom. In the Julian Bashir, Secret Agent holoprogram, they believed that the Soviets were involved in the disappearance of three secret agents in West Berlin. (DS9: "Change of Heart")
Background
The writers of the episode made a common mistake in differentiating between MI5 and MI6. Although the two intelligence bodies exist in today's world in the United Kingdom, MI5(aka "the Security Service") concerns only the internal security of the UK itself, while MI6 (aka "the Secret Intelligence Service")serves purposes of countries outside the UK, much like the FBI and CIA in the United States.However is not a police agency and it members do not have the power of arrest.