It is a requirement of the Constitutional Reform and Governance Act (PDF, 84 pages, 296KB) that selection for recruitment to the Civil Service must be on merit on the basis of fair and open competition. The Commission has a statutory duty to ensure that the merit requirement is upheld and is not being undermined. In upholding the merit requirement the Commission considers itself to be a guardian of principles and not of processes. 
The Commission's Recruitment Principles are the key source document to which departments and agencies must refer for the Commissioners' interpretation of the statutory requirement of appointment on merit on the basis of fair and open competition. The Recruitment Principles also outline the circumstances in which appointments may be made as exceptions to the merit requirement. Please see our Exceptions training material here
The Recruitment Principles are high level. It is for departments and agencies to develop and apply recruitment processes that best meet their business needs, while following the Recruitment Principles.
Civil Service Commissioners chair selection panels for all external recruitment competitions (i.e. a recruitment competition welcoming applications from outside the Civil Service) at Senior Civil Service pay band 2 (Directors), pay band 3 (Director General) and Permanent Secretary levels.
Commissioners also chair internal competitions at SCS pay band 3 and Permanent Secretary level under the terms of the Senior Appointments Protocol (PDF,  3 pages, 715KB), agreed with the Head of the Civil Service. 
The Civil Service Commissioners only chair open recruitment competitions for the most senior posts in the Civil Service. The Commission has a compliance and capability monitoring programme to provide assurance that selection for appointment to the Civil Service at other levels meets the statutory requirement.
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