For the purposes of summary dismissal, how does an employer legally define a serious breach of contract?

The contract of employment, and the disciplinary procedures made under it, should clearly state any breaches that will result in summary dismissal. This means that the contract or disciplinary procedure should define (providing a non-exhaustive list of examples) what constitutes "gross misconduct" for which an employee may expect to be dismissed without notice. The Acas code of practice on disciplinary and grievance procedures recommends that the disciplinary procedure allow for summary dismissal in cases of gross misconduct and that the procedure define which acts of misconduct will be classified as "gross" misconduct for these purposes.