If an employee is incapable of performing their job, either for medical reasons or due to lack of qualifications or ability, this can be a potentially fair reason for dismissal.
Except in serious cases or where the harm to the employer would be disproportionate, employers are expected to follow a reasonable procedure. Normally an informal approach is expected but if that doesn’t work then clear warnings and support should be given. Disciplinary meetings at which the employee is given certain protections (clear information and the right to be accompanied by a trade union representative or work colleague) under the ACAS Code (1).
The warnings may not apply in cases of gross negligence or where it is clear warnings would not work.