The Confined Spaces Code of Practice clearly states: Confined-spaces-Code of Practice 2019
Before any work in relation to a confined space starts, signs must be erected to prevent entry of persons not involved in the work.
Signs must warn against entry by people other than those who are listed on the confined space entry permit, and must be placed at each entrance to the confined space. Signs must be in place while the confined space is accessible, including when preparing to work in the space, during work in the space and when packing up on completion of the work.
Signposting alone should not be relied on to prevent unauthorised entry to a potential confined space. Security devices, for example locks and fixed barriers, should be installed.
Confined Space Risk Assessing