MI5 has no power to detain or arrest people. It is a civilian intelligence organisation rather than a law-enforcement agency. Those facts establish the legal boundary at the start: MI5 can contribute intelligence to cooperation with other bodies, but an arrest is not an action carried out under an MI5 power.
MI5 works with police and other law-enforcement agencies, and its intelligence can support their work. The relevant police or other law-enforcement body makes any arrest under its own statutory powers. The relationship can therefore bring intelligence into law-enforcement work while leaving the partner body responsible for deciding whether its power should be used.
What intelligence support contributes
Intelligence can inform the partner’s understanding and become part of the material it considers within its own responsibilities. Support does not prescribe the weight the partner must give that material or the result it must reach. The contribution is information for assessment, while the statutory decision remains a matter for the organisation carrying out the law-enforcement function.
The form of cooperation varies with the case and partner involved. A police force and another law-enforcement agency may have different responsibilities and processes for acting on intelligence. That variation affects the route through which intelligence enters the partner’s work. The sequence has to be understood from the particular case: which organisation received information, which assessed it and what action followed. This case-specific account describes the cooperation without treating the MI5 relationship as a standard chain of events.
Reading accounts of joint work
When an operation involves both MI5 and law enforcement, attribute each action to the organisation that performed it. Intelligence may explain how a matter reached the partner’s attention or informed its assessment. Any arrest belongs to the police or other law-enforcement body that decided to act and exercised its statutory power. Naming both the intelligence contribution and the arresting body keeps responsibility tied to the actor and legal basis even when the organisations work closely together.
