MI5 and the police have different purposes and legal powers. MI5 is a civilian intelligence organisation, not a law-enforcement agency, and its staff have no power to detain or arrest. Police investigate crime and use the statutory powers available in their jurisdiction. Cooperation between the organisations does not combine those roles.
In England and Wales, PACE Code G requires reasonable grounds to suspect involvement in an offence and reasonable grounds to believe that arrest is necessary. Suspicion addresses possible involvement in an offence; necessity asks whether arrest is required in the circumstances. Both questions belong to the police arrest assessment, and satisfying one does not remove the need to consider the other. Together they provide the two-part basis that police apply when assessing an arrest under Code G. The Code G test described here belongs to the England-and-Wales arrest framework; police elsewhere use the powers of their own jurisdiction.
Intelligence work and police cooperation
MI5 protects the UK against national-security threats by obtaining, analysing and assessing intelligence. It also works with partners to disrupt those threats. Obtaining, analysis and assessment describe how MI5 develops its understanding of a threat. Partner work allows that intelligence contribution to support disruption while leaving any separate statutory action with the body empowered to take it.
MI5 and the police cooperate and share intelligence, and they may work together to gather evidence for criminal proceedings. The organisations retain different functions throughout that cooperation. Intelligence sharing can inform each organisation’s work, while joint evidence gathering can support a criminal proceeding. MI5 contributes its intelligence function; the police remain responsible for investigating crime and applying the arrest powers available within their jurisdiction.
Oversight of intrusive powers
Intrusive investigatory powers are subject to statutory authorisation. That control concerns whether the proposed intrusive measure may be used. It is separate from the PACE assessment governing a police arrest, so identifying the power under consideration is necessary before identifying the applicable legal decision and decision-maker.
Where the double lock applies, an independent Judicial Commissioner also conducts a review. This adds independent review to the statutory authorisation process for the intrusive measure governed by that mechanism. An operation involving both an intrusive measure and an arrest therefore contains distinct legal decisions: authorisation and review for the measure, and the police assessment under the arrest power available in the relevant jurisdiction.
