Poor time-keeping is likely to constitute either a performance or a misconduct issue, and both are potentially fair reasons for dismissal. But you must still apply a fair procedure, which might include regular appraisals (with time-keeping issues being addressed), early warnings, providing the employee with a timescale for improvement, and obtaining medical evidence if there is any risk that the problem has a medical cause. If the reason for poor time-keeping is a disability, you may be obliged to make 'reasonable adjustments' to accommodate the employee, or you may be vulnerable to a disability discrimination claim.
