(depending on the rest of the sentence it might have to be added: "in case of non-compliance").
Greetings,

Nikolaus

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Note added at 2002-04-28 11:16:01 (GMT)
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Better: with the consequence of nullity in case of non-compliance, the sanctions have to be .....

is one of the possibilities

Op straffe van: under penalty of
nietigheid = nullity
nietigheidsverklaring: nullification, annulment

Ref.:

... is fourteen full years in males and twelve full years in females, under penalty of
nullity (unless natural puberty supplies the want of years). Marriages void ...
Beschrijving: (Catholic Encyclopedia)
Categorie: Society > Religion and Spirituality > ... > Reference > Catholic Encyclopedia > A
www.newadvent.org/cathen/01206c.htm

Depending on the target readership of your text, you might want to consider a relatively 'easy-reading' alternative to the rather heavy, legalistic (or even religious, as medabs suggests) tone of "under penalty of nullity", such as:

"A sanction will be invalid unless the employer, or his representative, has given proper notice to the person on whom it is imposed [/employee in question]."