If the performance issues are getting worse with an employee, follow the next step in your Capability or Disciplinary procedure.
This makes sure that you are maintaining consistency throughout your Company.
For example, an employee already with a First Written Warning, would be invited to a formal meeting and their performance would be discussed to highlight your ongoing concerns.
The outcome of that meeting may well be a Final Written Warning.
The employee must be made aware that a failure to improve will lead to their dismissal.