Many problems at work can be resolved informally, for example through an informal chat. An employee could raise a problem informally by telling their line manager or someone else at work, for example, another manager or someone in HR. It does not have to be in writing at this stage.
Take the problem seriously – if an employee raises a problem informally, you should still take it seriously. If you do not, the problem might be raised as a formal grievance later. A formal grievance might lead to the employee making a claim to an employment tribunal if it’s not resolved.