According to the 'continued business justification' principle in PRINCE2, a justifiable reason for a project is required before any work to start the project begins. This ensures that projects are initiated only when there is a clear business need, aligned with organizational objectives, and there is an expectation of value creation.
The business case is developed during the initial stages of the project and is maintained and reviewed throughout the project lifecycle.References: PRINCE2 Foundation Manual

During the Starting up a Project process, one of the key activities is to create role descriptions for the project management team. This ensures that everyone involved understands their responsibilities and the expectations placed upon them before the project commences. References: PRINCE2 Foundation Training Manual; PRINCE2 2017 Foundation handbook.

The purpose of a Project Brief in PRINCE2 is to provide a solid basis for the initiation of the project. The Project Brief includes the outline of the project's objectives, scope, constraints, and assumptions. It is developed during the 'Starting up a Project' process and is used to ensure that a clear, concise, and feasible plan is proposed before the project is formally initiated.
PRINCE2 7 Foundation resources and materials.