The Select Committee on the Constitution has investigated the Justice and Security Bill (HL) because the Bill would enable certain civil proceedings to be held under a closed material procedure (CMP). This is a constitutionally significant reform, challenging two principles of the rule of law: open justice and natural justice. Although the Government has amended its proposals for CMP, the Committee finds three basic flaws. Firstly, the Bill confers exclusive discretion on the Government. Second is the reduction of the courts’ capacity to ensure fairness in civil proceedings. The third flaw concerns the relation of the proposed CMP scheme to the current law and practice of public interest immunity (PII) and the exclusive discretion given to the Secretary of State: determining the more appropriate route to adopt in any particular litigation is the proper reserve of the court.