If you have been employed to do a specific task or are qualified to perform a specific task and only people who are qualified to perform that task and do it lawfully then if you lose that qualification for whatever reason, then your employer can dismiss you on the grounds of legal prohibition.

It is rarely used but some examples could include things like you are employed as a doctor in medicine and because of patient complaints you are struck of the medical list, then your employer can perfectly justify dismissing you on the grounds that you can no longer pursue your duties as a doctor because you are no longer legally entitled to do so.

Other examples would include being employed solely as a driver and losing your driving license, being banned from working as a teacher because of being convicted of an offence against children or being put on the at risk register in relation to children, young persons or old people.