In cases of more minor isolated transgressions or for potentially remediable problems, an appropriate disciplinary measure may well be to give the employee a warning.
A procedure on warnings is frequently part of an employer's disciplinary procedure and should follow the Advisory Conciliation and Arbitration Service (ACAS) Disciplinary Code. This specifies that one written and one final written warning should be given before dismissal. This procedure should normally be adhered to but in serious cases you may justifiably be able to proceed straight to a final warning (and the procedure should expressly state this).
Time between Warnings
There should be a reasonable time between the warnings so as to give the employee time to improve.
Where Warnings are Unnecessary
Gross misconduct may justify instant dismissal (without warnings or notice of termination). Examples of gross misconduct should ideally be specified in the employment contract to include violence, theft, gross disobedience, falsification of records and criminal offences which relate in some way to the employee's duties. However, such a list should be stated to be non-exhaustive, allowing you discretion where unanticipated transgressions are committed.
Failure to follow these rules has often led to employment tribunals finding that the dismissal has been procedurally unfair. Care should therefore be taken, particularly when dismissing employees who qualify for unfair dismissal protection.